qemu: Introduce code for blockdev-create

qemu: Introduce code for blockdev-create

QEMU finally exposes an interface which allows us to instruct it to
format or create arbitrary images. This is required for blockdev
integration of block copy and snapshots as we need to pre-format images
prior to use with blockdev-add.

This path introduces job handling and also helpers for formatting and
attaching a whole image described by a virStorageSource.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>