[9fans] Clearing venti arena during install

[bakul+plan9@bitblocks.com]

If fossil+venti is chosen during installation, the entire
venti arena partition is cleared.  Is there a way to avoid
clearing all of the arena space?  Since in a sense it is an
append only log, and the purpose for clearing it is,
presumably, to allow recreating the index, may be a shortcut
exists?  Doing so will save a lot of time and space
particularly on a qemu VM.  In qemu one can use a sparse file
or a qcow format file which allocates space only for data
actually written.  Writing even zeroes can eat up space.

Perhaps it is easier to add a do-not-zero option for install
as a fresh qemu disk is already "cleared".

Of course, it doesn't make much sense to use venti under qemu
but p9p does not run on all the platforms (notably windows).