Compilation on a RHEL 5 host failed, due to the older dbus headers
present on that machine, and triggered by commit 2aa167ca:

util/virdbus.c: In function 'virDBusMessageIterDecode':
util/virdbus.c:952: error: 'DBusBasicValue' undeclared (first use in this function)

* m4/virt-dbus.m4 (LIBVIRT_CHECK_DBUS): Check for DBusBasicValue.
* src/util/virdbuspriv.h (DBusBasicValue): Provide fallback.
Signed-off-by: NEric Blake <eblake@redhat.com>