qemu: capabilities: Detect active block commit via QMP schema probing if possible

qemu: capabilities: Detect active block commit via QMP schema probing if possible

For versions where we can probe that the arguments are optional we can
perform the probing by a schema query rather than sending a separate
command to do so.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>