KVM: selftests: Add KVM_SET_GUEST_DEBUG test

Covers fundamental tests for KVM_SET_GUEST_DEBUG. It is very close to the debug
test in kvm-unit-test, but doing it from outside the guest.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20200505205000.188252-4-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>