selftests: kvm: Add exception handling to selftests

Add the infrastructure needed to enable exception handling in selftests.
This allows any of the exception and interrupt vectors to be overridden
in the guest.

Signed-off-by: Aaron Lewis <aaronlewis@google.com>
Reviewed-by: Alexander Graf <graf@amazon.com>
Message-Id: <20201012194716.3950330-4-aaronlewis@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>