dracut in Fedora 20 doesn't know about *-pci modules (which were split
of *-hcd). So add them manually, in all flavors (ehci, ohci, xhci).
FixesQubesOS/qubes-issues#1517
Apparently add_drivers doesn't work. Looking at kernel-modules dracut
code, it can only be used for block-device driver and only makes sense
in --host-only mode.
So add additional module, which unconditionally install kernel modules.
