test: add PCI VirtIO transport unit tests

Add unit tests that ensure our logic for handling the interaction with
guest VirtIO drivers over the PCI transport is correct. Also, drop some
dead code and fix some of the handling of various fields.

A list of logic fixes we found while writing the tests:

* We found some effectively dead code. As VirtIO stipulates that
  accesses to 64bit configuration fields MUST happen with two individual
  32bit reads, so drop the code that was actually handling the 64bit
  accesses.
* There were fields that the code was handling as write-only, when the
  specification declares them read-write. This was not an issue, since
  the Linux driver apparently never reads them, but add the read
  handling, so that we are spec compliant.
* The specification mentions that the driver MUST NOT write a value of 0
  in the queue_enable field. Enforce it.

Signed-off-by: Babis Chalios <[email protected]>

Author: bchalios

src/vmm/src/devices/virtio/transport/mmio.rs

@@ -511,7 +511,7 @@ pub(crate) mod tests {
             }
         }
 
-        fn set_avail_features(&mut self, avail_features: u64) {
+        pub fn set_avail_features(&mut self, avail_features: u64) {
             self.avail_features = avail_features;
         }
     }