virFileRewrite: Move error reporting into callback

virFileRewrite: Move error reporting into callback

When rewriting a file using virFileRewrite() and error occurs
while writing into a temporary file it's actually the callback
that can report the most accurate error. Move error reporting
into very few callback we have currently. Those callbacks are
trivial so the benefit of this change is not obvious, but this
will change shortly when slightly more complicated callback is
introduced.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>