KVM: x86: allow 256 logical x2APICs again

While fixing an x2apic bug,
 17d68b76 KVM: x86: fix guest-initiated crash with x2apic (CVE-2013-6376)
we've made only one cluster available.  This means that the amount of
logically addressible x2APICs was reduced to 16 and VCPUs kept
overwriting themselves in that region, so even the first cluster wasn't
set up correctly.

This patch extends x2APIC support back to the logical_map's limit, and
keeps the CVE fixed as messages for non-present APICs are dropped.

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