iotests: Add more "skip_if_unsupported" statements to the python tests

The python code already contains a possibility to skip tests if the
corresponding driver is not available in the qemu binary - use it
in more spots to avoid that the tests are failing if the driver has
been disabled.

While we're at it, we can now also remove some of the old checks that
were using iotests.supports_quorum() - and which were apparently not
working as expected since the tests aborted instead of being skipped
when "quorum" was missing in the QEMU binary.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
4 files changed