Description of problem:
virt-manager should not create two guests with the save mac address on remote connection

Version-Release number of selected component (if applicable):
virt-manager-0.9.0-7.el6.x86_64
libvirt-0.9.4-19.el6.x86_64
kernel-2.6.32-207.el6.x86_64
qemu-kvm-0.12.1.2-2.196.el6.x86_64
python-virtinst-0.600.0-5.el6.noarch

How reproducible:
Always

Steps to Reproduce:
1. Add a remote connection:Virt-manager -> File -> add connection -> connect to remote host -> ssh + root + IP. then connect
2. On the remote connection, create a new guest "vm1" with default configuration.
3. On the remote connection, create another guest "vm2" with the same mac address with "vm1".
   Both of the two guest creatation are successful with no error.

Actual results:
Both of the two guest creatation are successful with no error.

Expected results:
There should be an error pop-up to tell us that "The MAC address you entered is already in use by another active virtual machine."

Additional info:
If i create two guest with same mac address on the localhost connection, there will be an error pop-up to say "The MAC address you entered is already in use by another active virtual machine."