As bhyve for a long time didn't have a notion of the explicit SATA
controller and created a controller for each drive, the bhyve driver
in libvirt acted in a similar way and didn't care about the SATA
controllers and assigned PCI addresses to drives directly, as
the generated command will look like this anyway:

 2:0,ahci-hd,somedisk.img

This no longer makes sense because:

 1. After commit c07d1c1c it's not possible to assign
    PCI addresses to disks
 2. Bhyve now supports multiple disk drives for a controller,
    so it's going away from 1:1 controller:disk mapping, so
    the controller object starts to make more sense now

So, this patch does the following:

 - Assign PCI address to SATA controllers (previously we didn't do this)
 - Assign disk addresses instead of PCI addresses for disks. Now, when
   building a bhyve command, we take PCI address not from the disk
   itself but from its controller
 - Assign addresses at XML parsing time using the
   assignAddressesCallback. This is done mainly for being able to
   verify address allocation via xml2xml tests
 - Adjust existing bhyvexml2{xml,argv} tests to chase the new
   address allocation

This patch is largely based on work of Fabian Freyer.