report error when virProcessGetStatInfo() is unable to parse data

report error when virProcessGetStatInfo() is unable to parse data

Currently virProcessGetStatInfo() always returns success and only logs error
when it is unable to parse the data. Make this function actually report the
error and return a negative value in this error scenario.

Fix the callers so that they do not override the error generated.
Also fix non-linux implementation of this function so as to report error.

Signed-off-by: Ani Sinha <ani@anisinha.ca>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>