GDBus: plug some memory leaks

GDBus: plug some memory leaks

These fixes makes udisks-daemon from udisks' gdbus-port branch, see

 http://cgit.freedesktop.org/udisks/log/?h=gdbus-port

handle 200 add/remove uevents generated by e.g.

 #!/bin/bash
 DEV=mmcblk0p1
 for n in `seq 200` ; do
     udevadm trigger --sysname-match=$DEV --action=remove
     udevadm trigger --sysname-match=$DEV --action=add
     echo foo $n
 done

without any substantial leaks.

Signed-off-by: David Zeuthen <davidz@redhat.com>