Set LIBVIRT_AUTOSTART=0 when running test suites

Occasionally some test cases will (accidentally) try to spawn
libvirtd. Set the LIBVIRT_AUTOSTART=0 environment variable to
ensure the remote driver never tries autostart.

tests/Makefile.am

@@ -244,6 +244,7 @@ TESTS_ENVIRONMENT =				\
   PATH="$(path_add)$(PATH_SEPARATOR)$$PATH"	\
   SHELL="$(SHELL)"				\
   LIBVIRT_DRIVER_DIR="$(abs_top_builddir)/src/.libs" \
+  LIBVIRT_AUTOSTART=0				\
   LC_ALL=C					\
   $(VG)