Implement job handling for the block copy job (drive/blockdev-mirror)
when using -blockdev. In contrast to the previously implemented
blockjobs the block copy job introduces new images to the running qemu
instance, thus requires a bit more handling.

When copying to new images the code now makes use of blockdev-create to
format the images explicitly rather than depending on automagic qemu
behaviour.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>