qmp-shell: don't show version greeting if unavailable

qemu-ga doesn't have greeting.
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20170504125432.21653-4-marcandre.lureau@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>

scripts/qmp/qmp-shell

@@ -286,6 +286,9 @@ class QMPShell(qmp.QEMUMonitorProtocol):
 
     def show_banner(self, msg='Welcome to the QMP low-level shell!'):
         print msg
+        if not self._greeting:
+            print 'Connected'
+            return
         version = self._greeting['QMP']['version']['qemu']
         print 'Connected to QEMU %d.%d.%d\n' % (version['major'],version['minor'],version['micro'])