usb-redir: Don't try to write to the chardev after a close event

Since we handle close async in a bh, do_write and thus write can get
called after receiving a close event. This patch adds a check to
the usb-redir write callback to not call qemu_chr_fe_write on a closed
backend.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
