Check "mac" address sooner in device_create function, before doing device_add.

Check "mac" address sooner in device_create function, before doing device_add.

In XendDomainInfo.py device_create function, when device type is
"vif", it has a paragraph to check the validity of "mac"
address. Before checking validity, device_add has been done. But after
checking validity, if the mac address is invlid, it raises VmError and
exits directly without doing clean work like removing the device item
from config info. This will cause that the incorrect mac address is
saved into VM Config file and VM fails to restart. If check "mac"
validity before doing device_add, there will be no problem.

Signed-off-by Chunyan Liu <CYLiu@novell.com>