domaincapstest: Allow tests of all capability variants

domaincapstest: Allow tests of all capability variants

Currently only the default variant ("") and "+hvf" are present in our
test data but upcoming patches will add another variant.

Upcoming test variants may not require any special handling so we should
be able to handle them using the default code path now that 'variant' is
properly propagated inside the test code.

Remove the restriction to test only the default ("") and "+hvf" variant
and modify the documentation to state that any other variant is tested
the same way as the default one.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>