x86: nVMX: Basic test of #DB intercept in L1

Test a single-step trap delivered by hardware to set expectations, and
then test a single-step trap synthesized by L0 to see if those
expectations are met. Single-step traps in simulated MOVSS-shadow are
tested as well.

As a bonus, test a single-step trap in a transactional region to set
expectations for that unusual case as well.

Signed-off-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Peter Shier <pshier@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>