cpu: Add helper cpu_exists(), to check if CPU with specified id exists

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
acpi_piix4: Add infrastructure to send CPU hot-plug GPE to guest

* introduce processor status bitmask visible to guest at 0xaf00 addr,
  where ACPI asl code expects it
* set bit corresponding to APIC ID in processor status bitmask on
  receiving CPU hot-plug notification
* trigger CPU hot-plug SCI, to notify guest about CPU hot-plug event

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>