Add interrupt handling code

Getting from host state to the guest is only half the story. We also need
to return to our host context and handle whatever happened to get us out of
the guest.

On PowerPC every guest exit is an interrupt. So all we need to do is trap
the host's interrupt handlers and get into our #VMEXIT code to handle it.

PowerPCs also have a register that can add an offset to the interrupt handlers'
adresses which is what the booke KVM code uses. Unfortunately that is a
hypervisor ressource and we also want to be able to run KVM when we're running
in an LPAR. So we have to hook into the Linux interrupt handlers.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>