KVM: arm64: Refactor __load_guest_stage2()

Refactor __load_guest_stage2() to introduce __load_stage2() which will
be re-used when loading the host stage 2.

Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Quentin Perret <qperret@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210319100146.1149909-24-qperret@google.com