newvm: Stop waiting for VM events if install fails

If `virt-install` fails before the VM starts for the first time; the
`virsh event` process running in the background will never terminate and
therefore the main process will `wait` forever.  We can avoid this by
killing the background process if `virt-install` fails.

newvm.sh

@@ -205,5 +205,5 @@ virsh metadata ${name} --live --config ${METADATA_XMLNS} dch \
     "<metadata><groups>${groups_xml}</groups></metadata>"
 ) &
 
-virt-install "$@"
+virt-install "$@" || kill $!
 wait