Add a logo to texi2pdf output. Other formats (info/html) are left as
future improvements.

The PDF (needed by texi2pdf for vectorized images) was generated from
pc-bios/qemu_logo.svg like this:

inkscape --export-pdf=docs/qemu_logo.pdf pc-bios/qemu_logo.svg

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20170113144135.5150-17-marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>