LXC: Image device should be reset in mount() and teardown()

When fixing bug#1208387, image device name is returned in function
setup_container. But it's missed to reset in function mount() and
teardown().

This patch is to fix it. We now just return the device name when
mounting disk image during setting up container and remove the
member variable 'device' from Class _DiskImage added in last patch.

Closes-bug: #1243108
Related-bug: #1208387

Change-Id: I2aaed6f3367edce4fb23b78f0e979440954609a8
Merge "LXC: Image device should be reset in mount() and teardown()"