virtio-serial: Add a 'virtserialport' device for generic serial port support

virtio-serial: Add a 'virtserialport' device for generic serial port support

This commit adds a simple chardev-based serial port. Any data the guest
sends is forwarded to the chardev and vice-versa.

Sample uses for such a device can be obtaining info from the guest like
the file systems used, apps installed, etc. for offline usage and
logged-in users, clipboard copy-paste, etc. for online usage.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>