iotests: Split 177 into two parts for compat=0.10

When originally written, test 177 explicitly took care to run
with compat=0.10.  Then I botched my own test in commit
81c219ac and f0a9c18f, by adding additional actions that require
v3 images.  Split out the new code into a new v3-only test, 204,
and revert 177 back to its original state other than a new comment.

Reported-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-id: 20180117165420.15946-2-eblake@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>