xl: use libxl_cpupoolinfo_list_free() in main_cpupoolnumasplit

xl: use libxl_cpupoolinfo_list_free() in main_cpupoolnumasplit

instead of manually freeing the elements of the list, which
is exactly the purpose of the said function.

Trade also a couple of 'return'-s with 'goto out'-s, which
is more in line with libxl usage paradigm.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <Ian.Jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: Juergen Gross <JGross@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>