p2v: Add usb-storage module and rebuild initrd (RHBZ#1157679).

Include the usb-storage kernel module, to permit booting from USB
keys.

Rerun dracut to rebuild the initramfs.  This is complicated because by
default dracut will try to build an initramfs for the running
(ie. appliance) kernel, which is completely useless.  We need to find
the latest installed kernel and rebuild the initramfs for that
instead, which is hairy.

This also does a small refactoring of the distro-specific code,
removing the $selinux_relabel variable and replacing it with a generic
$extra_args variable.

I only added this to the virt-builder script, since one assumes that
livecd-creator/whatever should make the kickstart ISO bootable by
installing whatever drivers and modules are necessary.