xm: Fix error message for xm create command.

When I tested xm create command, I saw the following error message.
I expected an error message "Error: (12, 'Cannot allocate memory')"
because I intentionally caused a memory shortage on the test.
But the error message was different from my expectation.

# xm create /xen/HVMdomain.1
Using config file "/xen/HVMdomain.1".
Error: an integer is required

I looked at xend.log to examine the cause why the error message was
shown.  (Could you see the attached xend.log?)
xend had the error message "Error: (12, 'Cannot allocate memory')"
first.  But xend changed the error message to "Error: an integer is
required" halfway.  I'm not sure about the cause why an exception
occurred in logging processing.  But when I applied an attached patch,
I confirmed that the error message that I expected was shown.  The
patch does not call xc.domain_destroy_hook() if self.domid is None.

Signed-off-by: Masaki Kanno <kanno.masaki@jp.fujitsu.com>

tools/python/xen/xend/XendDomainInfo.py

@@ -1773,7 +1773,8 @@ class XendDomainInfo:
 
         self._cleanupVm()
         if self.dompath is not None:
-            xc.domain_destroy_hook(self.domid)
+            if self.domid is not None:
+                xc.domain_destroy_hook(self.domid)
             self.destroyDomain()
 
         self._cleanup_phantom_devs(paths)