virstor: write large maps in chunks during label

During the initial label of a virstor device, write out the allocation
map in chunks if it is large (> 1 MB) in order to avoid large mallocs.

Even though the kernel virstor geom may still do a large malloc to
represent the allocation map, this may still be useful to avoid a
ulimit.

Reviewed by: markj
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D45517