libxl: Do not trust frontend for channel in getinfo

libxl: Do not trust frontend for channel in getinfo

libxl_device_channel_getinfo needs to examine devices without trusting
frontend-controlled data.  So:

* Use /libxl to find the backend path.
* Parse the backend path to find the backend domid, rather than
  reading it from the frontend.
* Tolerate FRONTEND/tty vanishing.

Note that there is a strange off-by-one error in the computation of
both fe_path and libxl_path in libxl_device_channel_getinfo: the
incoming channel->devid, which is copied to channelinfo->devid, has +1
applied to calculate the frontend path (and, after this patch, the
libxl path).  I.e., the devid passed to libxl_device_channel_getinfo
must be one less than the actual devid for the device being asked
about.

This is actually a bug which mirrors a bug in
libxl__append_channel_list, which fills in the devids of the channel
devices it finds with sequentially increasing numbers starting at 0.

In the usual case channels have real devids starting at 1 (because
there is the console, which is devid 0, but not a channel).  So these
bugs usually cancel out.

We do not address this problem at this time.  This bug does not have
any security implications.

This patch is part of XSA-175.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>