virttest: Call utils_net.update_mac_ip_address() in vm.create() when mac...

..._changeable set to yes

Sometime we directly call vm.create() to create a VM. mac_changeable could not work
in this situation. As it only used in qemu test so move utils_net.update_mac_ip_address()
to vm.create() function.

If other test need support mac_changeable later, we can call  update_mac_ip_address
in its start() function.

Signed-off-by: Feng Yang <fyang@redhat.com>