x86/debug: Explicitly write DR6 in the H/W watchpoint + DR6.BS sub-test

Explicitly set DR6.BS for the sub-test that verifies DR6.BS isn't cleared
when a data breakpoint (a.k.a. H/W watchpoint) #DB occurs.  Relying on
the single-step #DB tests to leave DR6 is all kinds of mean.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220120002923.668708-8-seanjc@google.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>