virsh iface-bridge: Ignore delay if stp is turned off
virsh iface-bridge: Ignore delay if stp is turned off

https://bugzilla.redhat.com/show_bug.cgi?id=892403

Delay only makes sense with STP enabled.
(cherry picked from commit a923865a0e0e7d8dc1bcd08a0b13a1a7e3cb20a7)

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