virfile: safezero: fall back to writing block by block if mmap fails

virfile: safezero: fall back to writing block by block if mmap fails

mmap can fail on 32-bit systems if we're trying to zero out a lot of data.
Fall back to using block-by-block writing in that case.  While we could map
smaller blocks it's unlikely that this code is used a lot and its easier to
just fall back to one of the existing methods.

Also modified the block-by-block zeroing to not allocate a megabyte of
zeroes if we're writing less than that.

Signed-off-by: Oskari Saarenmaa <os@ohmu.fi>