virttest.qemu_vm: But fix in add_pcidevice.

This patch will fix following issue:

"qemu: -device pci-assign,host=06:00.1,id=id_06:00.1:
 Parameter 'id' expects an identifier\n"

Also correct parameter in add_pcidevice function call.
Signed-off-by: NFeng Yang <fyang@redhat.com>