This patch allows us to distinguish between two
length values for each block:
    max_length - length of memory block that was allocated
    used_length - length of block used by QEMU/guest

Currently, we set used_length - max_length, unconditionally.
Follow-up patches allow used_length <= max_length.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>