apic: Drop APICCommonState.idx and use APIC ID as index in local_apics[]

apic: Drop APICCommonState.idx and use APIC ID as index in local_apics[]

local_apics[] is sized to contain all APIC ID supported in xAPIC mode,
so use APIC ID as index in it instead of constantly increasing counter idx.

Fixes error "apic initialization failed" when a CPU hotplugged and
unplugged more times than there are free slots in local_apics[].

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>