tools/libxl: Fix libxl__carefd_opened() to be more useful with an invalid fd

tools/libxl: Fix libxl__carefd_opened() to be more useful with an invalid fd

In the case that fd is -1, preserve errno and don't attempt to set
CLOEXEC.

Note that the implementation can still fail, as it ignores fcntl()
errors and may not set CLOEXEC properly.  Update the documentation
accordingly until it is fixed.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
---
Fixing the fnctl() error issue involves more TUITs than I currently
have.

New in v4