virsh: fix error messages in iface-bridge
virsh: fix error messages in iface-bridge

Bug: https://bugzilla.redhat.com/show_bug.cgi?id=878376

The error messages did not correspond to the attributes they printed.
(cherry picked from commit 924a6c7f6ad3fb3c3bb91feadccd12e6e471856f)

mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2