qemu: don't raise error upon interface update without <frames/> for <rx/> in coalesce

With this, incomplete XML without <frames/> for <rx/> in coalesce
won't raise error as before. It will leave the coalesce parameter
empty, thanks to passing it as a parameter and return an integer
to indicate error state - previously it returned pointer (or NULL
for both error and incomplete XML).
I also added a test case to test this functionality in the
qemuxml2xmltest.

The code went through some refactoring:
* change of a condition
* addition of a parameter
* change of order, that allowed removal of VIR_FREE
* removal of redundant labels and variables

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1535930
Signed-off-by: Kristina Hanicova <khanicov@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>

tests/qemuxml2argvdata/net-coalesce.xml

@@ -55,6 +55,15 @@
         </rx>
       </coalesce>
     </interface>
+    <interface type='network'>
+      <source network='default'/>
+      <mac address='52:54:00:e5:48:60'/>
+      <model type='virtio'/>
+      <coalesce>
+        <rx>
+        </rx>
+      </coalesce>
+    </interface>
     <serial type='pty'>
       <target port='0'/>
     </serial>

tests/qemuxml2xmloutdata/net-coalesce.xml

@@ -56,6 +56,12 @@
       <model type='virtio'/>
       <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
     </interface>
+    <interface type='network'>
+      <mac address='52:54:00:e5:48:60'/>
+      <source network='default'/>
+      <model type='virtio'/>
+      <address type='pci' domain='0x0000' bus='0x00' slot='0x07' function='0x0'/>
+    </interface>
     <serial type='pty'>
       <target type='isa-serial' port='0'>
         <model name='isa-serial'/>
@@ -68,7 +74,7 @@
     <input type='keyboard' bus='ps2'/>
     <audio id='1' type='none'/>
     <memballoon model='virtio'>
-      <address type='pci' domain='0x0000' bus='0x00' slot='0x07' function='0x0'/>
+      <address type='pci' domain='0x0000' bus='0x00' slot='0x08' function='0x0'/>
     </memballoon>
   </devices>
 </domain>