[Libguestfs] [PATCH v2 nbdkit 0/5] tests: Move common functions into tests/functions.sh

v1 was here: https://www.redhat.com/archives/libguestfs/2018-September/msg00057.html v2: - Fix tab vs spaces in configure.ac. - To generate list of plugins, use printf instead of xargs. - Use 'source ./functions.sh' instead of 'source functions'. - functions.sh: Consistent quoting in foreach_plugin function. - functions.sh: Change the contract of start_nbdkit so it requires (and checks that) the first two parameters are '-P pidfile'. This avoids needing to iterate over the arguments. - functions.sh: Use {1..10} instead of seq. - functions.sh: Add a common mechanism for handling traps / cleanups (this is extra patch 4/5). - functions.sh: start_nbdkit manages its own cleanup function for killing nbdkit on exit