x86/debug: Add framework for single-step #DB tests

Add a framework to the "debug" test for running single-step #DB tests,
future commits will extend the single-step tests to run in usermode and
to verify interaction with STI and MOVSS blocking.

Opportunistically add comments and stop open coding RFLAGS stuff.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220120002923.668708-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>