iotests: Fix 173

iotests: Fix 173

This test has been broken since 3.0.  It used TEST_IMG to influence
the name of a file created during _make_test_img, but commit 655ae6bb
changed things so that the wrong file name is being created, which
then caused _launch_qemu to fail.  In the meantime, the set of events
issued for the actions of the test has increased.

Why haven't we noticed the failure? Because the test rarely gets run:
'./check -qcow2 173' is insufficient (that defaults to using file protocol)
'./check -nfs 173' is insufficient (that defaults to using raw format)
so the test is only run with:
./check -qcow2 -nfs 173

Note that we already have a number of other problems with -nfs:
./check -nfs (fails 18/30)
./check -qcow2 -nfs (fails 45/76 after this patch, if exports does
not permit 'insecure')
and it's not on my priority list to fix those.  Rather, I found this
because of my next patch's work on tests using _send_qemu_cmd.

Fixes: 655ae6b
Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20191114213415.23499-2-eblake@redhat.com>