qapi: Add '@allow-write-only-overlay' feature for 'blockdev-snapshot'

qapi: Add '@allow-write-only-overlay' feature for 'blockdev-snapshot'

Anounce that 'blockdev-snapshot' command's permissions allow changing
of the backing file if the 'consistent_read' permission is not required.

This is useful for libvirt to allow late opening of the backing chain
during a blockdev-mirror.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200310113831.27293-8-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>