parallel: Don't access the global handle from multiple threads.

libguestfs handles are not thread safe, and it's not safe even to read
settings from the handle from multiple threads (eg. guestfs_get_trace).
Stop doing this in the parallel library.  This caused fairly
reproducible segfaults when you enabled '-x' and/or '-v'.

This fixes commit 34e77af1bf42a132589901164f29bd992b37589e.
launch: libvirt: Refactor SELinux warning code.

This is just code motion.