vhost: rename device id variable

vhost: rename device id variable

I failed to figure out what does "fh" mean here for a long while.
The only guess I could have had is "file handle". So, you get the
point that it's not well named.

I then figured it out that "fh" is derived from the fuse lib, and
my above guess is right. However, device_fh represents a virtio
net device ID. Therefore, here I rename it to vid (Virtio-net device
ID, or Vhost device ID; choose one you prefer) to make it easier for
understanding.

This name (vid) then will be considered to the only interface to
applications. That's another reason to do the rename: it's our
interface, make it more understandable.

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>