qga: Add 'guest-get-users' command

A command that will list all currently logged in users, and the time
since when they are logged in.

Examples:

virsh # qemu-agent-command F25 '{ "execute": "guest-get-users" }'
{"return":[{"login-time":1490622289.903835,"user":"root"}]}

virsh # qemu-agent-command Win2k12r2 '{ "execute": "guest-get-users" }'
{"return":[{"login-time":1490351044.670552,"domain":"LADIDA",
"user":"Administrator"}]}

Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>
* make g_hash_table_contains compat func inline to avoid
  unused warnings
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
diff --git a/include/glib-compat.h b/include/glib-compat.h
index 863c8cf..fcffcd3 100644
--- a/include/glib-compat.h
+++ b/include/glib-compat.h
@@ -217,6 +217,12 @@
 {
     g_hash_table_replace(hash_table, key, key);
 }
+
+static inline gboolean g_hash_table_contains(GHashTable *hash_table,
+                                             gpointer key)
+{
+    return g_hash_table_lookup_extended(hash_table, key, NULL, NULL);
+}
 #endif
 
 #ifndef g_assert_true