x86/debug: Add single-step #DB + STI/MOVSS blocking tests

Add a variety of test cases to verify single-step #DB interaction with
STI and MOVSS blocking.  Of particular note are STI blocking and MOVSS
blocking with DR7.GD=1, both of which require manual intervention from
the hypervisor to set vmcs.GUEST_PENDING_DBG_EXCEPTION.BS when
re-injecting an intercepted #DB with STI/MOVSS blocking active.

Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Alexander Graf <graf@amazon.de>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220120002923.668708-7-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>