libxl: Fix error handling in libxl__device_nic_from_xs_be

libxl: Fix error handling in libxl__device_nic_from_xs_be

Previously, this function would leak the temporary return from xs_read for
handle and mac address.  Fix both of these and the rest of the error handling.

This requires changing its return type and fixing the callers.

Introduce here a READ_BACKEND macro to make the code less repetitive.

Coverity ID: 1055886

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- spell out what the leaks were in the commit message ]