Memory block commands are only supported for linux with sysfs,
"guest-get-memory-block-info" was not in blacklist for other
cases.

Reported on:
https://bugzilla.redhat.com/show_bug.cgi?id=1751431



Signed-off-by: Basil Salman <bsalman@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>