replay: allow loading any snapshots before recording

This patch enables using -loadvm in recording mode to allow starting
the execution recording from any of the available snapshots.
It also fixes loading of the record/replay state, therefore snapshots
created in replay mode may also be used for starting the new recording.

Signed-off-by: Pavel Dovgalyuk <Pavel.Dovgaluk@ispras.ru>
Message-Id: <20180912081939.3228.56131.stgit@pasha-VirtualBox>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>