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.

Add virBhyveDriverCreateXMLConf, a simple wrapper around
virDomainXMLOptionNew that makes it easier to pass bhyveConnPtr
as a private data for parser. It will be used later for device
address allocation at parsing time.

Update consumers to use it instead of direct calls to
virDomainXMLOptionNew.

As we now have proper callbacks connected for the tests, update
test files accordingly to include the automatically generated
PCI root controller.

Currently, bhyvexml2xmltest does only one test. We can do better.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

The only implemented test for now is domain metadata test.