override: Properly override wrapper for virDomainGetGuestVcpus

Without the change to libvirt-override-api.xml generator.py would
generate the following function header:

    def guestVcpus(self, params, nparams, flags=0):

Since @params and @nparams are output-only in C and the python C
implementation actualy creates a dict from them we should not need to
pass them. Add the API definition to drop the two unnecessary args:

    def guestVcpus(self, flags=0):

The code did not work at all until this change as the C impl expects
only two arguments but the python required use of four.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1377071

libvirt-override-api.xml

@@ -698,5 +698,11 @@
       <arg name='flags' type='unsigned int' info='extra flags; not used yet, so callers should always pass 0'/>
       <return type='char *' info="dictionary of domain interfaces along with their MAC and IP addresses"/>
     </function>
+    <function name='virDomainGetGuestVcpus' file='python'>
+      <info>returns a dictionary containing information regarding guest vcpus</info>
+      <arg name='dom' type='virDomainPtr' info='pointer to the domain'/>
+      <arg name='flags' type='unsigned int' info='extra flags; not used yet, so callers should always pass 0'/>
+      <return type='int' info="dictionary of vcpu data returned by the guest agent"/>
+    </function>
   </symbols>
 </api>