KVM: Move declarations of memslot accessors up in kvm_host.h

Move the memslot lookup helpers further up in kvm_host.h so that they can
be used by inlined "to pfn" wrappers.

No functional change intended.

Tested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20241010182427.1434605-43-seanjc@google.com>