KVM: PPC: Book3S HV P9: Move vcpu register save/restore into functions

This should be no functional difference but makes the caller easier
to read.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211123095231.1036501-29-npiggin@gmail.com