KVM: x86/xen: interdomain evtchn support

KVM: x86/xen: interdomain evtchn support

Implement sending events between backend and the guest. To send an
event we mark the event channel pending by setting some bits in the
shared_info and vcpu_info pages and deliver the upcall on the
destination vcpu.

To send an event to dom0, we mark the event channel pending and
send an IPI to the destination vcpu, which would invoke the event
channel upcall handler, which inturn calls the ISR registered by
the backend drivers.

When sending to the guest we fetch the vcpu from the guest, mark the
event channel pending and deliver the interrupt to the guest.

Co-developed-by: Joao Martins <joao.m.martins@oracle.com>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>