Add initial CD-ROM support to VMD via vioscsi.

Add initial CD-ROM support to VMD via vioscsi.

* Adds 'cdrom' keyword to vm.conf(5) and '-r' to vmctl(8)
* Support various sized ISOs (Limitation of 4G ISOs on Linux guests)
* Known working guests: OpenBSD (primary), Alpine Linux (primary),
  CentOS 6 (secondary), Ubuntu 17.10 (secondary).
  NOTE: Secondary indicates some issue(s) preventing full/reliable
  functionality outside the scope of the vioscsi work.
* If the attached disks are non-bootable (i.e. empty), SeaBIOS (vmd's
  default BIOS) will boot from CD-ROM.

ok mlarkin@, jca@