migration: Improve QMP documentation

SQMP

migrate-set-cache-size

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Set cache size to be used by XBZRLE migration, the cache size will be rounded

down to the nearest power of 2

SQMP

query-migrate-cache-size

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Show cache size to be used by XBZRLE migration

                total amount in ms for downtime that was calculated on

                the last bitmap round (json-int)

- "ram": only present if "status" is "active", it is a json-object with the

  following RAM information (in bytes):

         - "transferred": amount transferred (json-int)

         - "remaining": amount remaining (json-int)

         - "total": total (json-int)

         - "duplicate": number of duplicated pages (json-int)

         - "normal" : number of normal pages transferred (json-int)

         - "normal-bytes" : number of normal bytes transferred (json-int)

  following RAM information:

         - "transferred": amount transferred in bytes (json-int)

         - "remaining": amount remaining to transfer in bytes (json-int)

         - "total": total amount of memory in bytes (json-int)

         - "duplicate": number of pages filled entirely with the same

            byte (json-int)

            These are sent over the wire much more efficiently.

         - "normal" : number of whole pages transfered.  I.e. they

            were not sent as duplicate or xbzrle pages (json-int)

         - "normal-bytes" : number of bytes transferred in whole

            pages. This is just normal pages times size of one page,

            but this way upper levels don't need to care about page

            size (json-int)

- "disk": only present if "status" is "active" and it is a block migration,

  it is a json-object with the following disk information (in bytes):

         - "transferred": amount transferred (json-int)

         - "remaining": amount remaining (json-int)

         - "total": total (json-int)

  it is a json-object with the following disk information:

         - "transferred": amount transferred in bytes (json-int)

         - "remaining": amount remaining to transfer in bytes json-int)

         - "total": total disk size in bytes (json-int)

- "xbzrle-cache": only present if XBZRLE is active.

  It is a json-object with the following XBZRLE information:

         - "cache-size": XBZRLE cache size

         - "bytes": total XBZRLE bytes transferred

         - "cache-size": XBZRLE cache size in bytes

         - "bytes": number of bytes transferred for XBZRLE compressed pages

         - "pages": number of XBZRLE compressed pages

         - "cache-miss": number of cache misses

         - "overflow": number of XBZRLE overflows

         - "cache-miss": number of XBRZRLE page cache misses

         - "overflow": number of times XBZRLE overflows.  This means

           that the XBZRLE encoding was bigger than just sent the

           whole page, and then we sent the whole page instead (as as

           normal page).

Examples:

1. Before the first migration

SQMP

migrate-set-capabilities

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Enable/Disable migration capabilities

- "xbzrle": xbzrle support

- "xbzrle": XBZRLE support

Arguments:

    },

SQMP

query-migrate-capabilities

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Query current migration capabilities