Xen: Bump minimum supported Xen version to 4.9

Xen: Bump minimum supported Xen version to 4.9

Platforms supported by libvirt have the following Xen versions

openSUSE Leap 15.2: 4.13
openSUSE Leap 15.3: 4.14
         Fedora 33: 4.14
      Ubuntu 18.04: 4.9
      Ubuntu 20.04: 4.11
     Debian Stable: 4.11

Bumping the minimum version doesn't allow us to drop much code, but it
does provide better alignment with libvirt's platform support statement.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>