docs: Update guestfs-performance with parallel scaling numbers and test script.
out-of-tree build: fix daemon

  CC     guestfsd-9p.o
In file included from ../../../daemon/9p.c:32:0:
../../../daemon/daemon.h:33:34: fatal error: guestfs-internal-all.h: No such file or directory
out-of-tree build: fix test-tool

(Not entirely sure whether using Gnulib to replace standard functions
is a good idea at all.)

link with libgnu:
  CCLD   libguestfs-test-tool
libguestfs_test_tool-test-tool.o: In function `main':
/home/bengen/src/deb/pkg-libvirt/libguestfs/debian/build-default/test-tool/../../../test-tool/test-tool.c:103: undefined reference to `rpl_getopt_long'
/home/bengen/src/deb/pkg-libvirt/libguestfs/debian/build-default/test-tool/../../../test-tool/test-tool.c:113: undefined reference to `rpl_optarg'
/home/bengen/src/deb/pkg-libvirt/libguestfs/debian/build-default/test-tool/../../../test-tool/test-tool.c:125: undefined reference to `rpl_optarg'
/home/bengen/src/deb/pkg-libvirt/libguestfs/debian/build-default/test-tool/../../../test-tool/test-tool.c:126: undefined reference to `rpl_optarg'
/home/bengen/src/deb/pkg-libvirt/libguestfs/debian/build-default/test-tool/../../../test-tool/test-tool.c:109: undefined reference to `rpl_optarg'
libguestfs_test_tool-test-tool.o: In function `set_qemu':
/home/bengen/src/deb/pkg-libvirt/libguestfs/debian/build-default/test-tool/../../../test-tool/test-tool.c:382: undefined reference to `rpl_perror'
/home/bengen/src/deb/pkg-libvirt/libguestfs/debian/build-default/test-tool/../../../test-tool/test-tool.c:366: undefined reference to `rpl_perror'
libguestfs_test_tool-test-tool.o: In function `make_files':
/home/bengen/src/deb/pkg-libvirt/libguestfs/debian/build-default/test-tool/../../../test-tool/test-tool.c:416: undefined reference to `rpl_perror'
/home/bengen/src/deb/pkg-libvirt/libguestfs/debian/build-default/test-tool/../../../test-tool/test-tool.c:428: undefined reference to `rpl_perror'