vmstate: Avoid seeking

vmstate: Avoid seeking

Seeking on vmstate save/load does not work if the underlying file is a
stream. We could try to make all QEMUFile* forward-seek-aware, but first
attempts in this direction indicated that it's saner to convert the few
qemu_fseek-on-vmstates users to plain reads/writes.

This fixes various subtle vmstate corruptions where unused fields were
involved.

Signed-off-by: Jan Kiszka <[email protected]>
Signed-off-by: Juan Quintela <[email protected]>
Signed-off-by: Anthony Liguori <[email protected]>