pc: acpi: create MADT.lapic entries only for valid lapics

do not assume that all lapics in range 0..apic_id_limit
are valid and do not create lapic entries for not
possible lapics in MADT.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
