docs: add qemu logo to pdf

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>

docs/qemu-ga-ref.texi

@@ -6,6 +6,10 @@
 
 @settitle QEMU Guest Agent Protocol Reference
 
+@iftex
+@center @image{docs/qemu_logo}
+@end iftex
+
 @copying
 This is the QEMU Guest Agent Protocol reference manual.
 

docs/qemu-qmp-ref.texi

@@ -6,6 +6,10 @@
 
 @settitle QEMU QMP Reference Manual
 
+@iftex
+@center @image{docs/qemu_logo}
+@end iftex
+
 @copying
 This is the QEMU QMP reference manual.