EC2: Disable PV support

Unfortunately, somewhere between 16.09 and 17.03, paravirtualized
instances stopped working. They hang at the pv-grub prompt
("grubdom>"). I tried reverting to a 4.4 kernel, reverting kernel
compression from xz to bzip2 (even though pv-grub is supposed to
support xz), and reverting the only change to initrd generation
(5a814747). Nothing worked so I'm
giving up.