builder: Remove recursive include of index-parse.h.

This breaks on Bison < 2.7, and seems unnecessary in any case since
the same file is included earlier.
builder: Disable internal parallel xzcat on RHEL 6.
virt-builder: Add --machine-readable option, as in virt-resize and virt-sparsify.

This makes it easier to query the virt-builder binary capabilities,
from external programs.