We need to keep DIR register unchanged across migration, but currently it
depends on the media_changed flags from block layer. Since we do not
save/restore it and the bdrv_open() called in dest node may set the
media_changed flag when trying to open floppy image, guest driver may think the
floppy have changed after migration. To fix this, a new filed media_changed in
FDrive strcutre was introduced in order to save and restore the it from block
layer through pre_save/post_load callbacks.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>