qga: flush explicitly when needed

According to the specification:
http://pubs.opengroup.org/onlinepubs/9699919799/functions/fopen.html

"the application shall ensure that output is not directly followed by
input without an intervening call to fflush() or to a file positioning
function (fseek(), fsetpos(), or rewind()), and input is not directly
followed by output without an intervening call to a file positioning
function, unless the input operation encounters end-of-file."

Without this change, an fwrite() followed by an fread() may lose the
previously written content, as shown in the following test.

Fixes:
https://bugzilla.redhat.com/show_bug.cgi?id=1210246

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
* don't confuse {write,read}() with f{write,read}() in
  commit msg (Laszlo)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
