savevm: Reset last block info at beginning of each save

If we save more than once we need to reset the last block info or else
only the first save has the actual block info and each subsequent save
will only use continue flags, making them unloadable independently.

Found-by: Miguel Di Ciurcio Filho <miguel.filho@gmail.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Glauber Costa <glommer@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
(cherry picked from commit 760e77ea)