prototype fast installer.

To install a distribution on a new volume, we take an existing
installed volume produced with sipb-xen-make-iso, and
 1. copy GRUB's data with dd
 2. frob the partition table with sfdisk, if the disk size is different
 3. make the filesystem
 4. twiddle the hostname and root password and whatever else
 5. make the swap area
 6. boot!

In this prototype, steps 2 and 4 aren't implemented yet;
steps 1, 3, and 5 all use numbers that come from what the
sipb-xen-make-iso etch installer does rather than from
actually reading the partition table; and the whole thing
is just a program in /usr/sbin, not integrated into what
remctl / the web app invoke.

It all takes about thirty seconds (28.0s most recently),
excluding the boot.

svn path=/trunk/packages/sipb-xen-guest-installer/sipb-xen-guest-installer/; revision=231