Force a flush when qemu_ftell is called.  This simplifies the buffer magic
(it also breaks qemu_ftell for input QEMUFiles, but we never use it).

Reviewed-by: Orit Wasserman <owasserm@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>