builder: Fedora: Enable Xen driver support.

See:
https://www.redhat.com/archives/libguestfs/2014-June/thread.html#00045

Notes:

(1) Edit dracut configuration to make sure the change is permanent, in
case kickstart, virt-builder or something else installs a kernel or
runs dracut.

(2) We need to run dracut on the just-installed kernel, not on the
running kernel.  The snippet to do this was copied from the Fedora
cloud kickstart.

Thanks: Olaf Hering, George Dunlap, Dario Faggioli.

builder/website/fedora.sh

@@ -60,6 +60,16 @@ poweroff
 %packages
 @core
 %end
+
+%post
+# Enable Xen domU support:
+pushd /etc/dracut.conf.d
+echo 'add_drivers+="xen:vbd xen:vif"' > virt-builder-xen-drivers.conf
+popd
+# Rerun dracut for the installed kernel (not the running kernel):
+KERNEL_VERSION=$(rpm -q kernel --qf '%{version}-%{release}.%{arch}\n')
+dracut -f /boot/initramfs-$KERNEL_VERSION.img $KERNEL_VERSION
+%end
 EOF
 
 # Clean up function.