vhost: declare device id as int

vhost: declare device id as int

device_fh repsents the device id for a specific virtio net device.
Firstly, "int" would be big enough: we don't need 64 bit. Secondly,
this could let us avoid the ugly "%" PRIu64 ".." stuff.

And since ctx.fh is derived from device_fh, declare it as int, too.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Tested-by: Rich Lane <rich.lane@bigswitch.com>
Acked-by: Rich Lane <rich.lane@bigswitch.com>