If booted from PXE, don't filter live devices

With other cases covered, stop filtering disks when we're PXE.
Assume that we're PXE if there's a BOOTIF argument, as this is
typically added by pxelinux and is not otherwise set

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=882846

Change-Id: I38c3316fcf7d9eb06f030b8ebc5319389278eca0
Signed-off-by: Ryan Barry <rbarry@redhat.com>