snapshot: qemu: Add support for external inactive snapshots

snapshot: qemu: Add support for external inactive snapshots

This patch adds support for external disk snapshots of inactive domains.
The snapshot is created by calling using qemu-img by calling:

 qemu-img create -f format_of_snapshot -o
 backing_file=/path/to/src,backing_fmt=format_of_backing_image
 /path/to/snapshot

in case the backing image format is known or probing is allowed and
otherwise:

 qemu-img create -f format_of_snapshot -o  backing_file=/path/to/src
 /path/to/snapshot

on each of the disks selected for snapshotting. This patch also modifies
the snapshot preparing function to support creating external snapshots
and to sanitize arguments. For now the user isn't able to mix external
and internal snapshots but this restriction might be lifted in the
future.