parallel: Propagate errors from worker threads and exit with error code.

In virt-df and virt-alignment-scan, ensure that errors that happen in
worker threads are propagated all the way up and result in
exit(EXIT_FAILURE).

Note that this makes the align/test-virt-alignment-scan-guests.sh test
fail (for a genuine reason).  This is fixed in the following commit.

This updates commit 8b90f55dc733e358a0892d8e6e5c67fe2cc4e891.
tests/guests: Add 'guests-all-good.xml{.in}' libvirt test guest set.

This differs from 'guests.xml' in that there are no guests with
missing disks, etc., so we can use this to test virt-alignment-scan
accurately.