lguest: use PGDIR_SHIFT for PAE code to allow different PAGE_OFFSET

We still assume the Guest and Host have the same PAGE_OFFSET settings,
but now we don't assume 0xC0000000.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Matias Zabaljauregui <zabaljauregui@gmail.com>