gitmodules: switch to qemu.org qboot mirror

gitmodules: switch to qemu.org qboot mirror

QEMU now hosts a mirror of qboot.git. QEMU mirrors third-party code to
ensure that users can always build QEMU even if the dependency goes
offline and so QEMU meets its responsibilities to provide full source
code under software licenses.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20200915130834.706758-2-stefanha@redhat.com>