xen: implement apic ipi interface

Map native ipi vector to xen vector.
Implement apic ipi interface with xen_send_IPI_one.

Tested-by: Steven Noonan <steven@uplinklabs.net>
Signed-off-by: Ben Guthro <ben@guthro.net>
Signed-off-by: Lin Ming <mlin@ss.pku.edu.cn>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>