Clean up filesystem unmounting in vmimage builds:

• Remove vm_umount_base function which is currently unused.
• Add umount_loop function which loops attempting to unmount one filesystem.
• Replace calls to umount with calls to umount_loop.
• Don't attempt to unmount ${DESTDIR}/dev if it isn't mounted.

The looping is necessary because sometimes umount fails due to filesystems
being busy. The most common cause of such busyness is periodic(8) jobs
running find / ....

Reviewed by: gjb