virtio-console: Prevent abort()s in case of host chardev close

A host chardev could close just before the guest sends some data to be
written.  This will cause an -EPIPE error.  This shouldn't be propagated
to virtio-serial-bus.

Ideally we should close the port once -EPIPE is received, but since the
chardev interface doesn't return such meaningful values to its users,
all we get is -1 for any kind of error.  Just return 0 for now and wait
for chardevs to return better error messages to act better on the return
messages.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
