core: set vm.uses_default_kernel=False when setting vm.kernel

Forgetting this leads to misterious errors (VM started with different
kernel than it was just set), so simplify the API.

Fixes QubesOS/qubes-issues#1400

core-modules/000QubesVm.py

@@ -526,6 +526,7 @@ class QubesVm(object):
                         "Kernel '%s' not properly installed: missing %s "
                         "file" % (new_value, f))
         self._kernel = new_value
+        self.uses_default_kernel = False
 
     @property
     def updateable(self):

qvm-tools/qvm-prefs

@@ -297,18 +297,16 @@ def set_kernel(vms, vm, args):
 
     kernel = args[0]
     if kernel == "default":
-        kernel = vms.get_default_kernel()
+        vm.kernel = vms.get_default_kernel()
         vm.uses_default_kernel = True
     elif kernel == "none":
-        kernel = None
-        vm.uses_default_kernel = False
+        vm.kernel = None
     else:
         if not os.path.exists(os.path.join(system_path["qubes_kernels_base_dir"], kernel)):
             print >> sys.stderr, "Kernel version {0} not installed.".format(kernel)
             return False
-        vm.uses_default_kernel = False
-
         vm.kernel = kernel
+
     return True
 
 def set_template(vms, vm, args):