libxl: libxl_tmem functions improving coding style

libxl: libxl_tmem functions improving coding style

In accordance with CODING_SYTLE:
 - Use 'r' for return values to functions whose return values are a
   different error space (like xc_tmem_control, xc_tmem_auth)

libxc functions are supposed to, on failure, set errno and always
return -1  which is the value stored in 'r', therfore use LOGE()
instead LOGEV() with the 'r' value.

Signed-off-by: Paulina Szubarczyk <paulinaszubarczyk@gmail.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>