xl: convert main() exit codes to EXIT_[SUCCESS|FAILURE]

xl: convert main() exit codes to EXIT_[SUCCESS|FAILURE]

Turning main() function exit codes towards using the EXIT_[SUCCESS|FAILURE]
constants, instead of instead of arbitrary numbers or libxl return codes.

Also includes a document comment in xl.h stating xl process should always
return EXIT_FOO and main_* can be treated as main() as if they are returning
a process exit status and not a function return value)

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>