https://bugzilla.redhat.com/show_bug.cgi?id=956994

Currently, it is possible to start an interface that is already running:

 # virsh iface-start eth2
 Interface eth2 started

 # echo $?
 0

 # virsh iface-start eth2
 Interface eth2 started

 # echo $?
 0

 # virsh iface-start eth2
 Interface eth2 started

 # echo $?
 0

Same applies for destroying a dead interface. We should not allow such
state transitions.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>