libxl: Clean up userlookup_helper_getpw* helper

libxl: Clean up userlookup_helper_getpw* helper

Bring conventions more in line with libxl__xs_read_checked():
- If found, return 0 and set pointer to non-NULL
- If not found, return 0 and set pointer to NULL
- On error, return libxl-style error number.

Update documentation to match.

Use CODING_STYLE compliant `r` rather than `ret`.

On error, log the error code before returning instead of discarding
it.

Now that it only returns 0 or errno, update caller error checks to be
`if (ret)` rather than `if (ret < 0)`.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>