snapshot: Add tests of virsh -c test:///default snapshot*

snapshot: Add tests of virsh -c test:///default snapshot*

Had this been in place earlier, I would have avoided the bugs in
commit 0baf6945 and 55c2ab3e. Writing the test required me to extend
the power of virsh - creating enough snapshots to cause fanout
requires enough input in a single session that adding comments and
markers makes it easier to check that output is correct. It's still a
bit odd that with test:///default, reverting to a snapshot changes the
domain from running to paused (possibly a bug in how the test driver
copied from the qemu driver) - but the important part is that the test
is reproducible, and any future tweaks we make to snapshot code have
less chance of breaking successful command sequences.

Signed-off-by: Eric Blake <eblake@redhat.com>
Acked-by: Michal Privoznik <mprivozn@redhat.com>