The capability is asserted when both block-stream and block-commit QMP
commands support the 'backing-mask-protocol' argument.
The argument causes qemu to record 'raw' as the backing file format in
case when a protocol node is used directly. This is needed to preserve
compatibility of images after a block-commit or block-pull libvirt
operation with older libvirt versions in case when we'll want to remove
the unneded 'raw' format drivers from the block graph.
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>