xl: improve main_tmem_* return codes

xl: improve main_tmem_* return codes

Functions libxl_tmem_freeze(), libxl_tmem_thaw(), libxl_tmem_set() and
libxl_tmem_shared_auth() located in libxl.c file return
ERROR_FAIL/ERROR_INVAL or internal error codes from libxc library
improve main_tmem_* return codes by returning EXIT_{SUCCESS/FAILURE}
accordingly to return codes of those functions.

Signed-off-by: Paulina Szubarczyk <paulinaszubarczyk@gmail.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>