libxl: Rename READ_BACKEND to READ_LIBXLDEV

libxl: Rename READ_BACKEND to READ_LIBXLDEV

We are going to want to change all the functions that use READ_BACKEND
to get untrustworthy information from the backend, to use trustworthy
information from /libxl.

This will involve replacing READ_BACKEND, which reads from be_path,
with a similar macro READ_LIBXLDEV, which reads from libxl_path.

The macro name change generates a lot of clutter in the diff.  So we
break it out into this separate patch.  Here, we rename the macro, but
the implementation does not really match the new name.

So, another way to look at this, is that we have transformed the bug:
 * All of the backends use READ_BACKEND, which is unsafe
into the new bug:
 * READ_LIBXLDEV actually reads be_path, which is unsafe.

There is no functional change as yet.

This is part of XSA-178.

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