add close callback for tty-based char device

add close callback for tty-based char device

v1 -> v2  coding style changes

Add a tty close callback. Right now if a guest device that is connected
to a tty-based chardev in the host is removed, the tty is not closed.
With this patch it is closed.

Example use case is connecting an emulated USB serial cable in the guest
to ttyS0 of the host using the monitor command:

usb_add serial::/dev/ttyS0

and then removing the device with:

usb_del serial::/dev/ttyS0

Signed-off-by: David Ahern <daahern@cisco.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>