dracut.sh: introduce "--no-hostonly-default-device" argument

dracut.sh: introduce "--no-hostonly-default-device" argument

Kdump doesn't need default host devices like root, swap, fstab, etc,
we only care about the dump target which can be added via "--mount"
or "--add-device". We met several issues that kdump kernel failed
due to one of those host devices added by dracut, additionally, the
needless devices(e.g. LVM) consume some appreciable amount of memory
which is more likely to cause OOM under memory-limited kdump.

So this patch introduced "--no-hostonly-default-device" to avoid
adding those default devices as host_devs.

Signed-off-by: Xunlei Pang <xlpang@redhat.com>