Making imageburn service async + removing unmounting + nicer error setting

BUG=chromium-os:15322,chromium-os:12226
TEST=Make sure imageburner works, insert removable device after
     reboot and run ./usr/sbin/image_burner_tester from shell
     (this will erase your device)

Change-Id: I03b737ae7439e71cb2fb98f576e5555a1fb8eaf5
Reviewed-on: http://gerrit.chromium.org/gerrit/896
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Tested-by: Toni Barzic <tbarzic@chromium.org>
7 files changed