x86/debug: Test IN instead of RDMSR for single-step #DB emulation test

Replace RDMSR with IN so that testing single-step #DBs on emulated
instructions can be run in userspace (by modifying IOPL).  IN is also
more interesting in that it is guaranteed to exit to host userspace,
whereas RDMSR will do so if and only if userspace is filtering the target
MSR.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220120002923.668708-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>