Allow setting "none" kernel to use in-vm kernel in any virt_mode

Automatically translate it to appropriate pvgrub flavor.

QubesOS/qubes-issues#5212

qubes/tests/vm/qubesvm.py

@@ -504,7 +504,7 @@ def test_220_include_in_backups(self):
 
     @unittest.mock.patch('qubes.config.qubes_base_dir', '/tmp/qubes-test')
     def test_250_kernel(self):
-        for kver in ("dummy", "dummy2"):
+        for kver in ("dummy", "dummy2", "pvgrub2", "pvgrub2-pvh"):
             kernel_dir = '/tmp/qubes-test/vm-kernels/' + kver
             os.makedirs(kernel_dir, exist_ok=True)
             open(os.path.join(kernel_dir, 'vmlinuz'), 'w').close()
@@ -517,6 +517,12 @@ def test_250_kernel(self):
             "dummy2")
         del vm.kernel
         self.assertPropertyDefaultValue(vm, 'kernel', "dummy")
+        vm.kernel = None
+        self.assertEqual(vm.kernel_path, "/tmp/qubes-test/vm-kernels/pvgrub2-pvh/vmlinuz")
+        vm.virt_mode = "pv"
+        self.assertEqual(vm.kernel_path, "/tmp/qubes-test/vm-kernels/pvgrub2/vmlinuz")
+        vm.virt_mode = "hvm"
+        self.assertIsNone(vm.kernel_path)
 
     @qubes.tests.skipUnlessDom0
     def test_251_kernel_invalid(self):

qubes/vm/qubesvm.py

@@ -1645,6 +1645,18 @@ def is_memory_balancing_possible(self):
     @property
     def kernel_path(self):
         if not self.kernel:
+            if self.virt_mode == 'pvh':
+                return os.path.join(
+                    qubes.config.qubes_base_dir,
+                    qubes.config.system_path['qubes_kernels_base_dir'],
+                    "pvgrub2-pvh",
+                    "vmlinuz")
+            if self.virt_mode == "pv":
+                return os.path.join(
+                    qubes.config.qubes_base_dir,
+                    qubes.config.system_path['qubes_kernels_base_dir'],
+                    "pvgrub2",
+                    "vmlinuz")
             return None
         return self.storage.kernels_dir + "/vmlinuz"
 
@@ -1663,9 +1675,9 @@ def is_kernel_from_vm(self):
         if self.virt_mode == 'hvm':
             return not self.kernel
         if self.virt_mode == 'pvh':
-            return self.kernel == 'pvgrub2-pvh'
+            return not self.kernel or self.kernel == 'pvgrub2-pvh'
         if self.virt_mode == 'pv':
-            return self.kernel == 'pvgrub2'
+            return not self.kernel or self.kernel == 'pvgrub2'
         assert False
 
     @property