qmp: Clean up capability negotiation after commit 02130314

qmp_greeting() offers capabilities to the client, and
qmp_qmp_capabilities() accepts or denies capabilities requested by the
client.  The two compute the set of available capabilities
independently.  Not nice.

Clean this up as follows.  Compute available capabilities just once in
monitor_qmp_caps_reset(), and store them in Monitor member
qmp.capab_offered[].  Have qmp_greeting() and qmp_qmp_capabilities()
use that.  Both are now oblivious of capability details.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180703085358.13941-31-armbru@redhat.com>