sipb-xen-losetup: a half-sane way to use losetup

You should use this instead of plain losetup; everything that uses this
will at least not have races with other things using this.

If it's missing a feature (e.g. a losetup argument) you want,
it shouldn't be hard to add.

svn path=/trunk/packages/sipb-xen-dom0/; revision=449