restapi: Fix guest nic configuration tag name

The initialization of a guest NIC should be represented using the "nic"
and "nics" tags, as follows:

  <nics>
    <nic>
      <name>eth0</name>
      ...
    </nic>
  </nics>

But due to a typo in the XML schema it is actually represented with
"nics" and "nics", as follows:

  <nics>
    <nics>
      <name>eth0</name>
      ...
    </nics>
  </nics>

This patch fixes the type so that the representation will be correct.

Change-Id: Ic0f0e543cb2905f5270263462aeb87afe852b250
Bug-Url: https://bugzilla.redhat.com/1062388
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>