build: Turn off warning about overlength-strings.

ISO apparently specified a limit of 4096 bytes/characters for strings.
The description of the guestfish command 'add-drive' is now longer
than this limit, causing this warning.

Turn off this warning, since GCC has no problems with these strings.
build: Fix 'make extra-tests' to call 'check-valgrind' instead of rule that doesn't exist.
build: Small rearrangement of Makefile.am check-slow rule.
build: Add 'make check-valgrind-with-appliance'.
Add support for: Gluster, Ceph (rbd), Sheepdog.

These are not tested at all, and so probably won't work.  However the
code and infrastructure is in place so we can fix bugs easily as they
arise.