Modify some of tests/test-vmstate.c due to qemu_bufopen() change.
If you create a QEMUSizedBuffer yourself, you have to explicitly
free it.

Signed-off-by: Yang Hongyang <yanghy@cn.fujitsu.com>
Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: Juan Quintela <quintela@redhat.com>
Cc: Amit Shah <amit.shah@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>