migration: remove use of qemu_bufopen from vmstate tests

migration: remove use of qemu_bufopen from vmstate tests

Some of the test-vmstate.c test cases use a temporary file
while others use a memory buffer. To facilitate the future
removal of the qemu_bufopen() function, convert all the tests
to use a temporary file.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <1461751518-12128-4-git-send-email-berrange@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>