storage: use root-cow.img for HVM template, to support reverting changes

We still can't support running HVM template and its VMs simultaneously
(easily), but still, have root-cow.img handled for HVM template, to
allow qvm-revert-template-changes.

Fixes QubesOS/qubes-issues#1573

core-modules/01QubesHVm.py

@@ -100,8 +100,6 @@ class QubesHVm(QubesResizableVm):
             (not 'xml_element' in kwargs or kwargs['xml_element'].get('guiagent_installed') is None):
             self.services['meminfo-writer'] = False
 
-        self.storage.rootcow_img = None
-
     @property
     def type(self):
         return "HVM"

core-modules/02QubesTemplateHVm.py

@@ -29,7 +29,7 @@ import stat
 import sys
 import re
 
-from qubes.qubes import QubesHVm,register_qubes_vm_class,dry_run
+from qubes.qubes import QubesHVm,register_qubes_vm_class,dry_run,vmm
 from qubes.qubes import QubesException,QubesVmCollection
 from qubes.qubes import system_path,defaults
 
@@ -96,7 +96,15 @@ class QubesTemplateHVm(QubesHVm):
     def commit_changes (self, verbose = False):
         self.log.debug('commit_changes()')
 
-        # nothing to do as long as root-cow.img is unused
-        pass
+        if not vmm.offline_mode:
+            assert not self.is_running(), "Attempt to commit changes on running Template VM!"
+
+        if verbose:
+            print >> sys.stderr, "--> Commiting template updates... COW: {0}...".format (self.rootcow_img)
+
+        if dry_run:
+            return
+
+        self.storage.commit_template_changes()
 
 register_qubes_vm_class(QubesTemplateHVm)

core/storage/xen.py

@@ -70,9 +70,12 @@ class XenStorage(QubesVmStorage):
                     "{root}:{rootcow}".format(
                         root=self.root_img, rootcow=self.rootcow_img),
                     "block-origin", self.root_dev, True)
-        elif self.vm.template and not self.vm.template.storage.rootcow_img:
-            # HVM template-based VM - template doesn't have own
-            # root-cow.img, only one device-mapper layer
+        elif self.vm.template and not hasattr(self.vm, 'kernel'):
+            # HVM template-based VM - only one device-mapper layer, in dom0 (
+            # root+volatile)
+            # HVM detection based on 'kernel' property is massive hack,
+            # but taken from assumption that VM needs Qubes-specific kernel (
+            # actually initramfs) to assemble the second layer of device-mapper
             return self.format_disk_dev(
                     "{root}:{volatile}".format(
                         root=self.vm.template.storage.root_img,