ch: Add support for `Unix` mode to serial port dev

ch: Add support for `Unix` mode to serial port dev

With Unix mode, pass a socket path to cloud-hypervisor.
Cloud-Hypervisor will attach guest's serial port to this socket path.
Users can connect to the serial port using one of the following commands:

`socat -,crnl UNIX-CONNECT:<path/to/socket>`

OR

`minicom --device unix#<path/to/socket>`

Signed-off-by: Praveen K Paladugu <prapal@linux.microsoft.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>