Plane: fixed is_flying() for VTOL flight

otherwise we may disarm mid-flight!

ArduPlane/is_flying.cpp

@@ -170,6 +170,9 @@ void Plane::update_is_flying_5Hz(void)
 bool Plane::is_flying(void)
 {
     if (hal.util->get_soft_armed()) {
+        if (quadplane.is_flying_vtol()) {
+            return true;
+        }
         // when armed, assume we're flying unless we probably aren't
         return (isFlyingProbability >= 0.1f);
     }

ArduPlane/quadplane.cpp

@@ -553,6 +553,15 @@ bool QuadPlane::should_relax(void)
     return relax_loiter;
 }
 
+// see if we are flying in vtol
+bool QuadPlane::is_flying_vtol(void)
+{
+    if (in_vtol_mode() && millis() - motors_lower_limit_start_ms > 5000) {
+        return true;
+    }
+    return false;
+}
+
 /*
   smooth out descent rate for landing to prevent a jerk as we get to
   land_final_alt. 

ArduPlane/quadplane.h

@@ -63,6 +63,9 @@ public:
     int8_t forward_throttle_pct(void);        
     float get_weathervane_yaw_rate_cds(void);
 
+    // see if we are flying from vtol point of view
+    bool is_flying_vtol(void);
+    
     struct PACKED log_QControl_Tuning {
         LOG_PACKET_HEADER;
         uint64_t time_us;