KVM: nVMX: Treat General Detect #DB (DR7.GD=1) as fault-like

KVM: nVMX: Treat General Detect #DB (DR7.GD=1) as fault-like

Exclude General Detect #DBs, which have fault-like behavior but also have
a non-zero payload (DR6.BD=1), from nVMX's handling of pending debug
traps.  Opportunistically rewrite the comment to better document what is
being checked, i.e. "has a non-zero payload" vs. "has a payload", and to
call out the many caveats surrounding #DBs that KVM dodges one way or
another.

Cc: Oliver Upton <oupton@google.com>
Cc: Peter Shier <pshier@google.com>
Fixes: 684c0422da71 ("KVM: nVMX: Handle pending #DB when injecting INIT VM-exit")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Link: https://lore.kernel.org/r/20220830231614.3580124-7-seanjc@google.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>