qemuxml2(argv|xml)test: Modernize 'shmem' test cases

Upgrade the relevant test cases to use latest capabilities. Note that
the 'shmem' (ivshmem) device is no longer supported and will be dropped
later.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

tests/qemuxml2argvdata/shmem-invalid-size.xml

@@ -18,6 +18,7 @@
     <controller type='pci' index='0' model='pci-root'/>
     <memballoon model='none'/>
     <shmem name='shmem0'>
+      <model type='ivshmem-plain'/>
       <size unit='K'>12345</size>
     </shmem>
   </devices>