x86: add io_apic_ops to allow interception

x86: add io_apic_ops to allow interception

Xen dom0 needs to paravirtualize IO operations to the IO APIC, so add
a io_apic_ops for it to intercept.  Do this as ops structure because
there's at least some chance that another paravirtualized environment
may want to intercept these.

[ Impact: indirect IO APIC access via io_apic_ops ]

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>