virttest.test_setup: Fix issue that 'pci-assign' is treated as not support in...

virttest.test_setup: Fix issue that 'pci-assign' is treated as not support in some host which really support it.

In some host pci-assign is aliased as kvm-pci-assign in qemu-kvm -device ?
So pci-assign is treated as not support. Add code to fix this.
Signed-off-by: NFeng Yang <fyang@redhat.com>