Introduce a new function virFileAllocate that will call the
non-destructive variants of safezero, essentially reverting
my commit 1390c268
    safezero: fall back to writing zeroes even when resizing
back to the state as of commit 18f03166
    virstoragefile: Have virStorageFileResize use safezero

This means that _ALLOCATE flag will no longer work on platforms
without the allocate syscalls, but it will not overwrite data
either.