KVM: selftests: Introduce UCALL_UNHANDLED for unhandled vector reporting

x86, the only arch implementing exception handling, reports unhandled
vectors using port IO at a specific port number. This replicates what
ucall already does.

Introduce a new ucall type, UCALL_UNHANDLED, for guests to report
unhandled exceptions. Then replace the x86 unhandled vector exception
reporting to use it instead of port IO.  This new ucall type will be
used in the next commits by arm64 to report unhandled vectors as well.

Tested: Forcing a page fault in the ./x86_64/xapic_ipi_test
	halter_guest_code() shows this:

	$ ./x86_64/xapic_ipi_test
	...
	  Unexpected vectored event in guest (vector:0xe)

Signed-off-by: Ricardo Koller <ricarkol@google.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210611011020.3420067-4-ricarkol@google.com