KVM: selftests: Add exception handling support for aarch64

KVM: selftests: Add exception handling support for aarch64

Add the infrastructure needed to enable exception handling in aarch64
selftests. The exception handling defaults to an unhandled-exception
handler which aborts the test, just like x86. These handlers can be
overridden by calling vm_install_vector_handler(vector) or
vm_install_exception_handler(vector, ec). The unhandled exception
reporting from the guest is done using the ucall type introduced in a
previous commit, UCALL_UNHANDLED.

The exception handling code is heavily inspired on kvm-unit-tests.

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/20210513002802.3671838-5-ricarkol@google.com