Change XBZRLE cache size in bytes (the size should be a power of 2, it will be
rounded down to the nearest power of 2).
If XBZRLE cache size is too small there will be many cache miss.

New query-migrate-cache-size QMP command and 'info migrate_cache_size' HMP
command to query cache value.
Signed-off-by: NBenoit Hudzia <benoit.hudzia@sap.com>
Signed-off-by: NPetter Svard <petters@cs.umu.se>
Signed-off-by: NAidan Shribman <aidan.shribman@sap.com>
Signed-off-by: NOrit Wasserman <owasserm@redhat.com>
Reviewed-by: NLuiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>