block: keep bs->total_sectors up to date even for growable block devices

If a BlockDriverState is growable, after every write we need to
check if bs->total_sectors might have changed.  With this change,
bdrv_getlength does not need anymore a system call.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>