tests: Drop various redundant tests

tests: Drop various redundant tests

All of these are either a subset of other tests, or provide
coverage for scenarios that are not really possible: for all
versions of QEMU that we support, the virt machine type has a
hard dependency on the generic PCIe controller, which means
that we will never need to fall back to virtio-mmio.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>