queue: Ensure the use of Descriptor accessors

Ensure that we're using the Descriptor accessors everywhere, instead
of accessing its fields directly.

This is needed for the next commit, where we'll be fixing the
endianess of each field on its accessor.

Signed-off-by: Sergio Lopez <slp@redhat.com>

Author: slp

crates/virtio-queue/src/descriptor.rs

@@ -68,7 +68,7 @@ impl Descriptor {
     /// If this is false, this descriptor is read only.
     /// Write only means the the emulated device can write and the driver can read.
     pub fn is_write_only(&self) -> bool {
-        self.flags & VIRTQ_DESC_F_WRITE != 0
+        self.flags() & VIRTQ_DESC_F_WRITE != 0
     }
 }