x86: VMX: Add another corner-case VMX-preemption timer test

Ensure that the delivery of a "VMX-preemption timer expired" VM-exit
doesn't disrupt single-stepping in the guest. Note that passing this
test doesn't ensure correctness.

Signed-off-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Oliver Upton <oupton@google.com>
Reviewed-by: Peter Shier <pshier@google.com>
Message-Id: <20200414001026.50051-2-jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>