qemu: disable external snapshot of readonly disk

qemu: disable external snapshot of readonly disk

Disable external snapshot of a readonly disk for domains as
this operation is not very useful. Such a snapshot is not
possible for active domains but the error message from QEMU
is more cryptic:

     error: internal error: unable to execute QEMU command 'transaction':
                         Could not create file: Permission denied

This error at least makes the error more understandable for
active domains and disallows for inactive domains as well.

Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>