qemu_snapshot: refactor validation of snapshot delete

qemu_snapshot: refactor validation of snapshot delete

Prepare the validation function for external snapshot delete support.

There is one exception when deleting `children-only` snapshots. If the
snapshot tree is like this example:

    snap1 (external)
     |
     +- snap2 (internal)
         |
         +- snap3 (internal)
             |
             +- snap4 (internal)

and user calls `snapshot-delete snap1 --children-only` the current
snapshot is external but all the children snapshots are internal only
and we are able to delete it.

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