Retry losetup -d for up to 10 seconds.

When partition tables are added to loopback devices, this can set off a
chain of udev hooks that may still be holding the loopback open. Failing
to detach loopback devices was the reason we were seeing leaked tmpfs
volumes.

Fixes bug #1178091

Change-Id: I836d6e2bbce824951dd4786e3ef28273ea18ee73
Merge "Retry losetup -d for up to 10 seconds."