Documentation: Add build support for documentation in pdf format

Documentation: Add build support for documentation in pdf format

Makefile already supported dvi, html and info formats,
but pdf was missing.

pdf is especially convenient for printing and for
documentation reviews. I hope it will help to
improve qemu's documentation.

Make now supports the new target 'pdf' which will
create qemu-doc.pdf and qemu-tech.pdf. It is also
possible to build both files individually.

texi2pdf and texi2dvi are rather noisy, so normally
some less important warnings are suppressed.
When make is called with V=1 (verbose mode),
warnings are not suppressed.

The patch also sorts the documentation targets
alphabetically and wraps a line which was too long.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>