vm-virtio: Implement console size config feature

One of the features of the virtio console device is its size can be
configured and updated. Our first iteration of the console device
implementation is lack of this feature. As a result, it had a
default fixed size which could not be changed. This commit implements
the console config feature and lets us change the console size from
the vmm side.

During the activation of the device, vmm reads the current terminal
size, sets the console configuration accordinly, and lets the driver
know about this configuration by sending an interrupt. Later, if
someone changes the terminal size, the vmm detects the corresponding
event, updates the configuration, and sends interrupt as before. As a
result, the console device driver, in the guest, updates the console
size.

Signed-off-by: A K M Fazla Mehrab <fazla.mehrab.akm@intel.com>

vm-virtio/src/lib.rs

@@ -143,4 +143,5 @@ pub enum Error {
     EpollCreateFd(io::Error),
     EpollCtl(io::Error),
     EpollWait(io::Error),
+    FailedSignalingDriver(io::Error),
 }