qemu.tests: Add enable_msix_vectors in hotplug_nic

The parameters of this function already updated. Need add this too.
Otherwise run this case with multi queues and enable_msix_vectors = yes
will failed.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #41 from ypu/nic_hotplug

qemu.tests: Add enable_msix_vectors in hotplug_nic