lguest: don't share Switcher PTE pages between guests.

We currently use the whole top PGD entry for the switcher, so we
simply share a fixed page of PTEs between all guests (actually, it's
one per Host CPU, to ensure isolation between guests).

Changes to a scheme where every guest has its own mappings.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>