block-common: Fix same_vm for no targets

same_vm is broken when the two main domains do not have targets.  otvm
and targetvm are both missing, which means they get set to -1 and then
converted to empty strings:

++10697+ local targetvm=-1
++10697+ local otvm=-1
++10697+ otvm=
++10697+ othervm=/vm/cc97bc2f-3a91-43f7-8fbc-4cb92f90b4e4
++10697+ targetvm=
++10697+ local frontend_uuid=/vm/844dea4e-44f8-4e3e-8145-325132a31ca5

The final comparison returns true since the two empty strings match:

++10697+ '[' /vm/844dea4e-44f8-4e3e-8145-325132a31ca5 = /vm/cc97bc2f-3a91-43f7-8fbc-4cb92f90b4e4 -o '' = /vm/cc97bc2f-3a91-43f7-8fbc-4cb92f90b4e4 -o /vm/844dea4e-44f8-4e3e-8145-325132a31ca5 = '' -o '' = '' ']'

Replace -1 with distinct strings indicating the lack of a value and
remove the collescing to empty stings.  The strings themselves will no
longer match, and that is correct.

++12364+ '[' /vm/844dea4e-44f8-4e3e-8145-325132a31ca5 = /vm/cc97bc2f-3a91-43f7-8fbc-4cb92f90b4e4 -o 'No target' = /vm/cc97bc2f-3a91-43f7-8fbc-4cb92f90b4e4 -o /vm/844dea4e-44f8-4e3e-8145-325132a31ca5 = 'No other target' -o 'No target' = 'No other target' ']'

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>

tools/hotplug/Linux/block-common.sh

@@ -112,14 +112,12 @@ same_vm()
                   "$FRONTEND_UUID")
   local target=$(xenstore_read_default  "/local/domain/$FRONTEND_ID/target"   \
                  "-1")
-  local targetvm=$(xenstore_read_default "/local/domain/$target/vm" "-1")
+  local targetvm=$(xenstore_read_default "/local/domain/$target/vm" "No Target")
   local otarget=$(xenstore_read_default  "/local/domain/$otherdom/target"   \
                  "-1")
   local otvm=$(xenstore_read_default  "/local/domain/$otarget/vm"   \
-                 "-1")
-  otvm=${otvm%-1}
-  othervm=${othervm%-1}
-  targetvm=${targetvm%-1}
+                 "No Other Target")
+
   local frontend_uuid=${FRONTEND_UUID%-1}
   
   [ "$frontend_uuid" = "$othervm" -o "$targetvm" = "$othervm" -o \