VirtualDisk: Rework provisioning and validation behavior

We separate all the provisioning bits to diskbackend.py. VirtualDisk
users now need to explicitly opt in to storage creation by using
set_create_storage().

validation is no longer done automatically, users must call the
validation() command.

__init__ drops all extra parameters.

This will eventually get us to a point where we can unify the manual
XML building and XML parsing machinery, and get consistent validation
behavior between devices.

tests/xmlparse-xml/change-disk-out.xml

@@ -18,10 +18,10 @@
   <vcpu>5</vcpu>
   <devices>
     <emulator>/usr/lib/xen/bin/qemu-dm</emulator>
-    <disk type="file" device="disk">
-      <source file="/dev/loop0"/>
+    <disk type="block" device="disk">
       <target dev="hda" bus="ide"/>
       <serial>frob</serial>
+      <source dev="/dev/loop0"/>
       <driver name="test" type="raw"/>
     </disk>
     <disk type="block" device="disk">

tests/xmlparse-xml/change-media-out.xml

@@ -15,10 +15,10 @@
       <driver name="qemu" type="qcow2"/>
       <source file="/default-pool/default-vol"/>
     </disk>
-    <disk type="file" device="cdrom">
+    <disk type="block" device="cdrom">
       <target dev="hdb" bus="ide"/>
-      <driver name="qemu" type="qcow2"/>
-      <source file="/disk-pool/diskvol1"/>
+      <driver name="qemu" type="raw"/>
+      <source dev="/disk-pool/diskvol1"/>
     </disk>
     <disk type="block" device="cdrom">
       <target dev="hdc" bus="ide"/>