qemu_snapshot: create: don't require disk-only flag for offline external snapshot

qemu_snapshot: create: don't require disk-only flag for offline external snapshot

Historically creating offline external snapshot required disk-only flag
as well. Now when user requests new snapshot for offline VM and at least
one disk is specified to use external snapshot we will no longer require
disk-only flag as all other not specified disk will use external
snapshots as well.

Resolves: https://issues.redhat.com/browse/RHEL-22797
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>