tools/libxc: pass errno to callers of xc_domain_save

tools/libxc: pass errno to callers of xc_domain_save

Callers of xc_domain_save use errno to print diagnostics if the call
fails. But xc_domain_save does not preserve the actual errno in case of
a failure.

This change preserves errno in all cases where code jumps to the label
"out". In addition a new label "exit" is added to catch also code which
used to do just "return 1".

Now libxl_save_helper:complete can print the actual error string.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>