KVM: x86: use physical LAPIC array for logical x2APIC

Logical x2APIC IDs map injectively to physical x2APIC IDs, so we can
reuse the physical array for them.  This allows us to save space by
sizing the logical maps according to the needs of xAPIC.

Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>