KVM: PPC: Book3S HV P9: Split !nested case out from guest entry

The differences between nested and !nested will become larger in
later changes so split them out for readability.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Fabiano Rosas <farosas@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220303053315.1056880-5-npiggin@gmail.com