x86: nVMX: Add some corner-case VMX-preemption timer tests

Verify that both injected events and debug traps that result from
pending debug exceptions take precedence over a "VMX-preemption timer
expired" VM-exit resulting from a zero-valued VMX-preemption timer.

Signed-off-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Oliver Upton <oupton@google.com>
Reviewed-by: Peter Shier <pshier@google.com>
Message-Id: <20200414001026.50051-1-jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>