mllib: Create config.ml from configure and use it for --version options.

Also don't internationalize the output of the --version option, as
that isn't useful.
build: xz is required.

It is needed for libguestfs-make-fixed-appliance and many other places.
builder: Don't run virt-resize when not necessary.

If:
 - the output is a regular file
 - the output format is raw
 - the user didn't specify the --size option
then we don't need to run virt-resize.  Simply uncompress
the template directly to the output file.
builder: Make xzcat binary configurable and use AC_PATH_PROG.
builder: Use pxzcat (optionally) to speed up xzcat step.