xl: improve return and exit codes of parse related functions

xl: improve return and exit codes of parse related functions

Turning  parsing related functions exit codes towards using the
EXIT_[SUCCESS|FAILURE] constants, instead of instead of arbitrary numbers
or libxl return codes.
        - for main_*: arbitrary -> EXIT_SUCCESS|EXIT_FAILURE.
        - for internal fucntion: arbitrary -> 0/1.

Don't touch parse_config_data() which is big enough to deserve its own patch.

Signed-off-by: Harmandeep Kaur <write.harmandeep@gmail.com>
Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>