Enable partitions for lguest block device

The lguest block device only requests one minor, which means
partitions don't work (eg "root=/dev/lgba1").

Let's follow the crowd and ask for 16.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>