dist: Speed up distribution compression

This almost reverts b7200d72.  The size is increased from 11M to 13M
and the compression is sped up from 2 minutes to 17 seconds.  The
compression level is removed because -9 doesn't allow multiple threads
to be spawned.  Effectively speeds up distcheck as well.
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

Makefile.am

@@ -22,7 +22,7 @@ GENHTML = genhtml
 SUBDIRS = . gnulib/lib include/libvirt src daemon tools docs gnulib/tests \
   tests po examples
 
-XZ_OPT ?= -9ev
+XZ_OPT ?= -v -T0
 export XZ_OPT
 
 ACLOCAL_AMFLAGS = -I m4