uuid/label based fstab during bsdinstall

Greetings, everyone.

I am testing FreeBSD 11.0-BETA3 release on a virtual machine (qemu,
with
edkII ovmf).

Currently, bsdinstall creates device path based block device driver
scheme instead of uuid/label based device scheme.

bsdinstall-generated fstab has some drawback. For example, when
switching from ide to virtio on qemu, freebsd can't find its root
partition because the path name has changed from 'da0' to 'vtbd0'.

I suggest adding an option during bsdinstall to select fstab block
device pointer scheme. User will choose a scheme based on fs-uuid,
fs-label, geom label (glabel), gpt id, gpt label, or driver based
numbering scheme (da0/vtbd0 style).

If fstab scheme choice is too hard to implement, it would be better to
just switch default fstab generation to label based scheme, so that
FreeBSD kernel will be able to find its rootfs in different
circumstances.

I hope this will be implemented soon.