Do not attempt to run hook with an unknown vnic

If hotunplugNic() is called with a badly-specified vnic, we should not
run the after_nic_hotunplug_fail() hook, as we cannot supply its
required param: nicXml is unassigned at this stage.

Change-Id: I0e4f1d8e0b7f83324edd903f9204bb05fdd19626
Bug-Url: https://bugzilla.redhat.com/919356
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/12891
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Assaf Muller <amuller@redhat.com>
Reviewed-by: Itzik Brown <itzikb@mellanox.com>
Reviewed-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Tested-by: Assaf Muller <amuller@redhat.com>