builder: virt-install now requires '--serial pty' parameter.

This updates commit 73ebaa30668ce7d9e3e413c3abf67a74829dfe5d.
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: Update Fedora 20 image, with Xen drivers.

This commit also removes the disk image signature.  These have not
been needed since virt-builder 1.24.1.  No one should be running that
version of virt-builder since there are newer 1.24 branch versions.
builder: Get rid of remaining disk image signatures.

These were deprecated and replaced in virt-builder 1.24.1.