use absolute URLs for .gitmodules

The relative URLs do not work when cloning a fork of qemu or when
cloning from the Savannah URL.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

.gitmodules

 [submodule "roms/vgabios"]
 	path = roms/vgabios
-	url = ../vgabios.git
+	url = git://git.qemu.org/vgabios.git/
 [submodule "roms/seabios"]
 	path = roms/seabios
-	url = ../seabios.git
+	url = git://git.qemu.org/seabios.git/