vm-virtio: Reset underlying device on driver request

If the driver triggers a reset by writing zero into the status register
then reset the underlying device if supported. A device reset also
requires resetting various aspects of the queue.

In order to be able to do a subsequent reactivate it is required to
reclaim certain resources (interrupt and queue EventFDs.) If a device
reset is requested by the driver but the underlying device does not
support it then generate an error as the driver would not be able to
configure it anyway.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vm-virtio/src/queue.rs

@@ -256,6 +256,12 @@ impl Queue {
         min(self.size, self.max_size)
     }
 
+    /// Reset the queue to a state that is acceptable for a device reset
+    pub fn reset(&mut self) {
+        self.ready = false;
+        self.size = self.max_size;
+    }
+
     pub fn is_valid(&self, mem: &GuestMemoryMmap) -> bool {
         let queue_size = self.actual_size() as usize;
         let desc_table = self.desc_table;