qemu: capabilities: Introduce QEMU_CAPS_BLOCKDEV_BACKUP

qemu: capabilities: Introduce QEMU_CAPS_BLOCKDEV_BACKUP

Upcoming commit will enable full backup support (incremental part
requires blockdev-reopen, which won't happen in qemu for at least
another release).

Add a capability that the 'blockdev-backup' job is supported by qemu
capped, but limited to when qemu supports QEMU_CAPS_BLOCKDEV.

We can also use it in the expression to enable
QEMU_CAPS_INCREMENTAL_BACKUP since it's a pre-requisite too.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>