tests: Add tests for qcow2 comapt option when converting images

Since qemuimg.convert() uses utils.watchCmd instead of utils.execCmd,
FakeExecCmd was modified so it can patch both functions.

This patch is separate from the actual fix, to allow easy backporting of
the actual fix, without backporing the testing infrastructure, which is
missing in older versions.

Change-Id: I0d19b59bf4880e20f53369c79ae34386d274915d
Relates-To: https://bugzilla.redhat.com/1139707
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/33350
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>