qemu: snapshot: Introduce helpers for creating overlays on <transient/> disks

qemu: snapshot: Introduce helpers for creating overlays on <transient/> disks

To implement <transient/> disks we'll need to install an overlay on top
of the original disk image which will be discarded after the VM is
turned off. This was initially implemented by qemu but libvirt never
picked up this option as the overlays were created by qemu without
libvirt involvment which didn't work with SELinux.

With blockdev the qemu feature became unsupported so we need to do this
via the snapshot code anyways.

The helpers introduced in this patch prepare a fake snapshot disk
definition for a disk which is configured as <transient/> and use it to
create a snapshot (without actually modifying metadata or persistent
def).

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Tested-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Tested-by: Ján Tomko <jtomko@redhat.com>