Use qemu-project.org domain name

qemu.org is held by a third-party and no core community contributor has
access to the DNS configuration.  This leaves the website exposed to
outages due to DNS issues or IP address changes.  For example, if the
web server IP address needs to change we cannot guarantee qemu.org will
point to it!

The newer qemu-project.org domain name is owned by Anthony Liguori
<anthony@codemonkey.ws>.  You can confirm this by querying the whois
information.  Also note that the #qemu IRC channel topic already
references qemu-project.org.

Short of having a dedicated legal entity to hold the domain name on
behalf of the community, qemu-project.org seems like the safest bet.

Let's replace references to qemu.org with qemu-project.org.

Note that git-submodule(1) does not detect URL changes.  The following
commands clear out and re-initialize all submodules to ensure you are
using the latest URLs:

  $ git submodule deinit . # you'll be warned if you have local changes
  $ rm -rf .git/modules...