iotests: Fix 051 for compat=0.10

iotests: Fix 051 for compat=0.10

051 has both compat=1.1 and compat=0.10 tests (once it uses
lazy_refcounts, once it tests that setting them does not work).
For the compat=0.10 tests, it already explicitly creates a suitable
image.  So let's just ignore the user-specified compat level for the
lazy_refcounts test and explicitly create a compat=1.1 image there, too.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20171123020832.8165-12-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>