Fixes the following error when attempting to add a disk with bus='virtio'
to a machine which actually supports virtio-mmio (caught with ARM virt):

virtio disk cannot have an address of type 'virtio-mmio'

The problem has been likely introduced by
e8d55172. Before that
qemuAssignDevicePCISlots() was never called for ARM "virt" machine.
Signed-off-by: NPavel Fedin <p.fedin@samsung.com>