subtests.cfg.sample: Introduce virt_test_type

Introduce an additional param, virt_test_type,
that is a list of virt backends where the given
test works on, for example:

virt_test_type = kvm libvirt

Means the test can run under the kvm and libvirt
tests, while

virt_test_type = libvirt

Means the test is libvirt exclusive.

This will be used to filter available tests,
and help to fail incompatible tests.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>