urlfetcher: Force a flush after writing to a file

While downloading really small files, on some file systems, the files
may not be flushed on time for whatever reason they've been downloaded.

This issue was noticed after trying to perform unattended installations
and noticing that some files, particularly really small ones, where just
empty.

While the original issue would be fixed by doing the flush on
_HTTPURLFetcher::_write(), let's also force it on _URLFetcher::_write()
to do the same.

Reviewed-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>

virtinst/install/urlfetcher.py

@@ -156,6 +156,7 @@ class _URLFetcher(object):
             fileobj.write(buff)
             total += len(buff)
             self.meter.update(total)
+        fileobj.flush()
         return total
 
     def _grabber(self, url):
@@ -291,6 +292,7 @@ class _HTTPURLFetcher(_URLFetcher):
             fileobj.write(data)
             total += len(data)
             self.meter.update(total)
+        fileobj.flush()
         return total