xenapi: Resolve Coverity REVERSE_INULL

Coverity notes in xenapiDomainGetXMLDesc that 'vms' is dereferenced
a few times before a "if (vms) xen_vm_set_free(vms);" call is made.
Since we'd exit out much sooner if the fetch of the vms failed, just
remove the unnecessary "if (vms)" check.

src/xenapi/xenapi_driver.c

@@ -1563,8 +1563,7 @@ xenapiDomainGetXMLDesc(virDomainPtr dom, unsigned int flags)
         }
         xen_vif_set_free(vif_set);
     }
-    if (vms)
-        xen_vm_set_free(vms);
+    xen_vm_set_free(vms);
     xml = virDomainDefFormat(defPtr, flags);
     virDomainDefFree(defPtr);
     return xml;