KVM: x86: reset APIC ID when enabling LAPIC

APIC ID should be set to the initial APIC ID when enabling LAPIC.
This only matters if the guest changes APIC ID.  No sane OS does that.

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