qemu_file: use fwrite() correctly

fwrite() returns the number of items written.  But when there is one
error, it can return a short write.

In the particular bug that I was tracking, I did a migration to a
read-only filesystem.  And it was able to finish the migration
correctly.  fwrite() never returned a negative error code, nor zero,
always 4096. (migration writes chunks of about 14000 bytes).  And it
was able to "complete" the migration with success (yes, reading the
file was a bit more difficult).

To add insult to injury, if your amount of memory was big enough (12GB
on my case), it overwrote some important structure, and from them,
malloc failed.  This check makes the problem go away.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
