ppc: allow certain HV interrupts to be delivered to guests

ppc hypervisors have delivered system reset and machine check exception
interrupts to guests in some situations (e.g., see FWNMI feature of LoPAPR,
or NMI injection in QEMU).

These exceptions are architected to set the HV bit in hardware, however
when injected into a guest, the HV bit should be cleared. Current code
masks off the HV bit before setting the new MSR, however this happens after
the interrupt delivery model has calculated delivery mode for the exception.
This can result in the guest's MSR LE bit being lost.

Account for this in the exception handler and don't set HV bit for guest
delivery.

Also add another sanity check to ensure similar bugs get caught.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
