conf: net: Fix deadlock if assignment of network def fails
conf: net: Fix deadlock if assignment of network def fails

https://bugzilla.redhat.com/show_bug.cgi?id=921777

When the assignment fails, the network object is not unlocked and next
call that would use it deadlocks.
(cherry picked from commit f8230891243f86e920d04a0751512cc31055ff8c)

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