Historically we've only supported the <backingStore> as an output-only
element for domain disks. The documentation states that it may become
supported on input. To allow management apps detectin once that happens
add a domain capability which will be asserted if the hypervisor driver
will be able to obey the <backingStore> as configured on input.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>