lguest: remove NOTIFY call and eventfd facility.

Disappointing, as this was kind of neat (especially getting to use RCU
to manage the address -> eventfd mapping).  But now the devices are PCI
handled in userspace, we get rid of both the NOTIFY hypercall and
the interface to connect an eventfd.

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