bhyve: add virBhyveDriverCreateXMLConf

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.

tests/bhyvexml2xmloutdata/bhyvexml2xmlout-base.xml

@@ -19,6 +19,7 @@
       <target dev='hda' bus='sata'/>
       <address type='drive' controller='0' bus='0' target='2' unit='0'/>
     </disk>
+    <controller type='pci' index='0' model='pci-root'/>
     <controller type='sata' index='0'/>
     <interface type='bridge'>
       <mac address='52:54:00:b9:94:02'/>