build: increase xz compression level

Increase the default compression level to 9 from 6.

This also increases decompression memory requirements
from 9 MB to 65 MB.

Also turn on verbosity.

Makefile.am

@@ -22,6 +22,9 @@ GENHTML = genhtml
 SUBDIRS = . gnulib/lib include/libvirt src daemon tools docs gnulib/tests \
   tests po examples
 
+XZ_OPT ?= -9ev
+export XZ_OPT
+
 ACLOCAL_AMFLAGS = -I m4
 
 EXTRA_DIST = \