iotests/279: Fix for non-qcow2 formats

iotests/279: Fix for non-qcow2 formats

First, driver=qcow2 will not work so well with non-qcow2 formats (and
this test claims to support qcow, qed, and vmdk).

Second, vmdk will always report the backing file format to be vmdk.
Filter that out so the output looks like for all other formats.

Third, the flat vmdk subformats do not support backing files, so they
will not work with this test.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20191219144243.1763246-1-mreitz@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>