qemu: Fix pci_hotplug testing

Turns out commit e9953f8a doesn't include the definition
of the function vm.is_supported_device, making this test
to fail. As a fix, instead of adding yet another vm method,
add a function local to the test, at least for now. This
function could be turned to an API in case it looks and
works OK to the other reviewers, although I'd like to
avoid API proliferation.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>