x86/apic: Add test to verify aliased xAPIC IDs both receive IPI

Verify that multiple vCPUs with the same physical xAPIC ID receive an
IPI sent to said ID.  Note, on_cpu() maintains its own CPU=>ID map and
is effectively unusuable after changing the xAPIC ID.  Update each vCPU's
xAPIC ID from within the IRQ handler so as to avoid having to send
yet another IPI from vCPU0 to tell vCPU1 to update its ID.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221001011301.2077437-10-seanjc@google.com