qemu_snapshot: prepare data for external snapshot deletion

qemu_snapshot: prepare data for external snapshot deletion

In order to save some CPU cycles we will collect all the necessary data
to delete external snapshot before we even start. They will be later
used by code that deletes the snapshots and updates metadata when
needed.

With external snapshots we need data that libvirt gets from running QEMU
process so if the VM is not running we need to start paused QEMU process
for the snapshot deletion and kill at afterwards.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>