libxl: set channel devid when not provided by application

libxl: set channel devid when not provided by application

Applications like libvirt may not populate a device devid field,
delegating that to libxl. If needed, the application can later
retrieve the libxl-produced devid. Indeed most devices are handled
this way in libvirt, channel devices included.

This works well when only one channel device is defined, but more
than one results in

qemu-system-i386: -chardev socket,id=libxl-channel-1,\
path=/tmp/test-org.qemu.guest_agent.00,server,nowait:
Duplicate ID 'libxl-channel-1' for chardev

Besides the odd '-1' value in the id, multiple channels have the same
id, causing qemu to fail. A simple fix is to set an uninitialized
devid (-1) to the dev_num passed to libxl__init_console_from_channel().

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>