Adding POWER9 cpu model to cpu_map.xml

As POWER9 model is not available in cpu_map.xml virsh capabilities
donot display the cpu model and vendor details. This patch
provides those details

src/cpu/cpu_map.xml

@@ -1571,6 +1571,11 @@
       <pvr value='0x004d0000' mask='0xffff0000'/>
     </model>
 
+    <model name='POWER9'>
+      <vendor name='IBM'/>
+      <pvr value='0x004e0000' mask='0xffff0000'/>
+    </model>
+
     <!-- Freescale-based CPU models -->
     <model name='POWERPC_e5500'>
       <vendor name='Freescale'/>