Rather than just picking the first CD (or failing that, HDD) we come
across, if the user has picked a boot device ordering with <boot
order=''>, respect that (and just try to boot the lowest-index device).

Adds two sets of tests to bhyve2xmlargv; 'grub-bootorder' shows that we
pick a user-specified device over the first device in the domain;
'grub-bootorder2' shows that we pick the first (lowest index) device.

We still default to bhyveloader(1) if no explicit bootloader
configuration is supplied in the domain.

If the /domain/bootloader looks like grub-bhyve and the user doesn't
supply /domain/bootloader_args, we make an intelligent guess and try
chainloading the first partition on the disk (or a CD if one exists,
under the assumption that for a VM a CD is likely an install source).

Caveat: Assumes the HDD boots from the msdos1 partition. I think this is
a pretty reasonable assumption for a VM. (DrvBhyve with Bhyveload
already assumes that the first disk should be booted.)

I've tested both HDD and CD boot and they seem to work.

 - docs/formatstorage.html.in: document 'zfs' pool type, add it
   to a list of pool types that could use source physical devices
 - docs/storage.html.in: update a ZFS pool example XML with
   source physical devices, mention that starting from 1.2.9 a
   pool could be created from this devices by libvirt and in earlier
   versions user still has to create a pool manually
 - docs/drvbhyve.html.in: add an example with ZFS pools

 - mention that one disk and one network limitation
   is no longer current for 1.2.6 and newer
 - add 'cdrom' device to the sample domain XML

- Document 'domxml-to-native' command
- Mention that the nmdm console support needs an appropriate
  kernel module loaded

* Add nmdm type device to domain format documnetation
* Add a section about nmdm console usage to the bhyve driver
  documentation