KVM: x86/xen: Inject vCPU upcall vector when local APIC is enabled

KVM: x86/xen: Inject vCPU upcall vector when local APIC is enabled

Linux guests since commit b1c3497e604d ("x86/xen: Add support for
HVMOP_set_evtchn_upcall_vector") in v6.0 onwards will use the per-vCPU
upcall vector when it's advertised in the Xen CPUID leaves.

This is injected through the local APIC as an MSI, as opposed to the
older system vector which was merely injected by the hypervisor any
time the CPU was able to receive an interrupt and the upcall_pending
flags is set in its vcpu_info.

Effectively, that makes it edge triggered instead of level triggered.

We lose edges.

Specifically, when the local APIC is *disabled*, delivering the MSI
will fail. Xen checks the vcpu_info->evtchn_upcall_pending flag when
enabling the local APIC for a vCPU and injects the vector immediately
if so.

Since userspace doesn't get to notice when the guest enables a local
APIC which is emulated in KVM, KVM needs to do the same.

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