Remove too strict checking for kernel on startup

Nowadays empty kernel is automatically converted to pvgrub2, so don't
refuse that value.

Fixes: ba8bc65 "Allow setting "none" kernel to use in-vm kernel in any virt_mode"

QubesOS/qubes-issues#5212

qubes/vm/qubesvm.py

@@ -1188,9 +1188,6 @@ async def start(self, start_guid=True, notify_function=None,
                                 f'not available'
                             )
 
-                if self.virt_mode == 'pvh' and not self.kernel:
-                    raise qubes.exc.QubesException(
-                        'virt_mode PVH require kernel to be set')
                 await self.storage.verify()
 
                 if self.netvm is not None: