Virttest.qemu vm modify pci bus name and spapr vlan params

For ppc64 and x86, the pci bus name is different as qtree shows,
in qtree, the pci bus named 'pci' for ppc64, but 'pci.0' for x86.

As a result, it will cause an error if use 'pci.0' for ppc64.

At the same time, ppc64 qemu use spapr-vlan for nic device,
and this device is belongs to bus 'spapr-vio-bus' not 'pci'.

The two patches are to solve the problems.

Signed-off-by: Mike Qiu <qiudayu@linux.vnet.ibm.com>