xenapi: enable block device access for stack user

Although nova is setting the permissions on block devices, sometimes it
fails, and that results in an instance failing to launch. It is only an
issue for 3-part images, and images accessed through block devices. This
patch adds an udev rule, so that devices will be accessible.

fixes bug 1218251

Change-Id: I837ea515457fbfc50e9ce138ea9de9db12baa8be
Merge "xenapi: enable block device access for stack user"