block/xen-blkback: Make it running on 64KB page granularity

The PV block protocol is using 4KB page granularity. The goal of this
patch is to allow a Linux using 64KB page granularity behaving as a
block backend on a non-modified Xen.

It's only necessary to adapt the ring size and the number of request per
indirect frames. The rest of the code is relying on the grant table
code.

Note that the grant table code is allocating a Linux page per grant
which will result to waste 6OKB for every grant when Linux is using 64KB
page granularity. This could be improved by sharing the page between
multiple grants.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Acked-by: "Roger Pau Monné" <roger.pau@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>