qemu: snapshot: error out early when reverting snapshot for VM with non-file disk

qemu: snapshot: error out early when reverting snapshot for VM with non-file disk

Before this patch the code would start the revert process by destroying
the VM and preparing to revert where it would fail with following error:

    error: unsupported configuration: source for disk 'sdb' is not a regular file; refusing to generate external snapshot name

and leaving user with offline VM even if it was running.

Make the check before we start the revert process to not destroy VMs.

Resolves: https://issues.redhat.com/browse/RHEL-30971
Resolves: https://issues.redhat.com/browse/RHEL-79928
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>