KVM: arm64: Improve error handling from check_host_shared_guest()

KVM: arm64: Improve error handling from check_host_shared_guest()

The check_host_shared_guest() path expects to find a last-level valid
PTE in the guest's stage-2 page-table. However, it checks the PTE's
level before its validity, which makes it hard for callers to figure out
what went wrong.

To make error handling simpler, check the PTE's validity first.

Signed-off-by: Quentin Perret <qperret@google.com>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20250207145438.1333475-2-qperret@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>