Save exit code $? at the beginning of the END block

Otherwise cleaning up by doing some system() calls garbles $? with
unwanted values.

KNOWN_BUGS.markdown

@@ -4,8 +4,7 @@ KNOWN BUGS in xen-tools
 Bugs to fix before next release
 -------------------------------
 
-* Exit code of a failed xen-create-image run seems still to be 0, at
-  least in some cases.
+[None so far]
 
 Bugs to fix rather soon
 -----------------------

bin/xen-create-image

@@ -4094,7 +4094,10 @@ sub clean_up () {
 
 END
 {
-    exit 0 if $VERSION || $HELP || $MANUAL || $DUMPCONFIG;
+    # Capture exit code
+    my $exitcode = $?;
+
+    exit $exitcode if $VERSION || $HELP || $MANUAL || $DUMPCONFIG;
 
     my $host_rsa_key = '';
     #
@@ -4234,4 +4237,6 @@ END
         }
         logprint("\n");
     }
+
+    exit $exitcode;
 }