qemu: caps: Add capability for blockdev-add/blockdev-del

qemu: caps: Add capability for blockdev-add/blockdev-del

The capability also represents that 'blockdev-add' is functional. It's
necessary to detect it via presence of 'blockdev-del' since blockdev-add
did not have the unsupported 'x-blockdev-add' version previously and
thus would be marked as present even if we could not use it.

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