i386/xen: Add support for Xen event channel delivery to vCPU

i386/xen: Add support for Xen event channel delivery to vCPU

The kvm_xen_inject_vcpu_callback_vector() function will either deliver
the per-vCPU local APIC vector (as an MSI), or just kick the vCPU out
of the kernel to trigger KVM's automatic delivery of the global vector.
Support for asserting the GSI/PCI_INTX callbacks will come later.

Also add kvm_xen_get_vcpu_info_hva() which returns the vcpu_info of
a given vCPU.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>