virttest.qemu_vm: Remove init_pci_addr from VM.

Now vm.init_pci_addr is effected by previous vm.

We set init_pci_addr to 3 in multi_disk.max_disk, but this case still
fail for:
VMPCIOutOfRangeError: Too many PCI devices added on vm 'virt-tests-vm1',
 max supported '31'

because previous case use default value 4.

As init_pci_addr only used in get_free_pci_addr, so remove it from vm
 and only get init_pci_addr in get_free_pci_addr().
Also set default for init_pci_addr to 3.

Signed-off-by: Feng Yang <fyang@redhat.com>