Add migrate_set_cache_size command

Add migrate_set_cache_size command

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: Benoit Hudzia <[email protected]>
Signed-off-by: Petter Svard <[email protected]>
Signed-off-by: Aidan Shribman <[email protected]>
Signed-off-by: Orit Wasserman <[email protected]>
Reviewed-by: Luiz Capitulino <[email protected]>
Reviewed-by: Eric Blake <[email protected]>