bhyve: implement support for commandline args

Implement support for passing custom command line arguments
to bhyve using the 'bhyve:commandline' element:

  <bhyve:commandline>
    <bhyve:arg value='-newarg'/>
  </bhyve:commandline>

 * Define virDomainXMLNamespace for the bhyve driver, which
   at this point supports only the 'commandline' element
   described above,
 * Update command generation code to inject these command line
   arguments between driver-generated arguments and the vmname
   positional argument.

Signed-off-by: Roman Bogorodskiy <bogorodskiy@gmail.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>

tests/bhyvexml2xmloutdata/bhyvexml2xmlout-commandline.xml

@@ -0,0 +1,37 @@
+<domain type='bhyve' xmlns:bhyve='http://libvirt.org/schemas/domain/bhyve/1.0'>
+  <name>bhyve</name>
+  <uuid>df3be7e7-a104-11e3-aeb0-50e5492bd3dc</uuid>
+  <memory unit='KiB'>219136</memory>
+  <currentMemory unit='KiB'>219136</currentMemory>
+  <vcpu placement='static'>1</vcpu>
+  <os>
+    <type arch='x86_64'>hvm</type>
+    <boot dev='hd'/>
+  </os>
+  <clock offset='utc'/>
+  <on_poweroff>destroy</on_poweroff>
+  <on_reboot>restart</on_reboot>
+  <on_crash>destroy</on_crash>
+  <devices>
+    <disk type='file' device='disk'>
+      <driver name='file' type='raw'/>
+      <source file='/tmp/freebsd.img'/>
+      <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'>
+      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
+    </controller>
+    <interface type='bridge'>
+      <mac address='52:54:00:b9:94:02'/>
+      <source bridge='virbr0'/>
+      <model type='virtio'/>
+      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
+    </interface>
+  </devices>
+  <bhyve:commandline>
+    <bhyve:arg value='ARGUMENT1'/>
+    <bhyve:arg value='ARGUMENT2'/>
+  </bhyve:commandline>
+</domain>