libxl: Do not trust frontend for vusb

libxl: Do not trust frontend for vusb

Do not use the frontend directory for enumerating the vusb devices;
since the frontend could delete them, this could result in devices
being lost and not torn down, etc.  Instead, use the /libxl directory
for enumeration.  So:

* Replace vusb_be_from_xs_fe with vusb_be_from_xs_libxl
* Change the call sites
* Change various places that use the dompath to use libxl_dom_path
* Rename some `path' variables appropriate (to spot any missed updates)
* Parse backend domid out of backend path rather than reading it from
  the frontend (several places)

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
---
v3: Whitespace adjustment to parameter list indentation
v2: New patch, following rebase