xen: introduce a new way to setup event channel upcall

The main differences with the currently implemented method are:

• Requires a local APIC EOI, since it doesn't bypass the local APIC as the previous method used to do.
• Can be set to use different IDT vectors on each vCPU. Note that FreeBSD doesn't make use of this feature since the event channel IDT vector is reserved system wide.

Note that the old method of setting the event channel upcall is
not removed, and will be used as a fallback if this newly introduced
method is not available.

MFC after: 1 month
Sponsored by: Citrix Systems R&D