appmenus: force X-Qubes-VM category for all VM-related entries

This will ease filtering entries when constructing applications menu.
For example '<OnlyUnallocated/>' key used in Xfce4 before looks to
introduce some problems.

Fixes QubesOS/qubes-issues#2129

appmenus-files/qubes-appmenu-select.desktop

@@ -7,4 +7,4 @@ Terminal=false
 Name=%VMNAME%: Add more shortcuts...
 GenericName=%VMNAME%: Add more shortcuts...
 StartupNotify=false
-Categories=System;
+Categories=System;X-Qubes-VM;

appmenus-files/qubes-dispvm-firefox.desktop

@@ -7,4 +7,4 @@ Terminal=false
 Name=DispVM: Firefox web browser
 GenericName=DispVM: Web browser
 StartupNotify=false
-Categories=Network;
+Categories=Network;X-Qubes-VM;

appmenus-files/qubes-start.desktop

@@ -7,4 +7,4 @@ Terminal=false
 Name=%VMNAME%: Start
 GenericName=%VMNAME%: Start
 StartupNotify=false
-Categories=System;
+Categories=System;X-Qubes-VM;

appmenus-scripts/qubes-receive-appmenus

@@ -213,6 +213,9 @@ def create_template(path, values):
         if key in values:
             desktop_entry += "{0}=%VMNAME%: {1}\n".format(key, values[key])
 
+    # force category X-Qubes-VM
+    values["Categories"] = values.get("Categories", "") + "X-Qubes-VM;"
+
     for key in ["Comment", "Categories"]:
         if key in values:
             desktop_entry += "{0}={1}\n".format(key, values[key])