KVM: arm64: Convert kvm_set_spte_hva() to generic page-table API

Convert kvm_set_spte_hva() to use kvm_pgtable_stage2_map() instead
of stage2_set_pte().

Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Quentin Perret <qperret@google.com>
Link: https://lore.kernel.org/r/20200911132529.19844-9-will@kernel.org