virttest.arcg: add new interface get_qemu_pci_bus_name

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.

This patch is to get the exact pci bus name for different arches.

Signed-off-by: Mike Qiu <qiudayu@linux.vnet.ibm.com>