nsis: Improved support for parallel installation of 32 and 64 bit code

32 and 64 bit variants of QEMU already had different default installation
directories, but used a common registry key for saving the choosen
directory. This is confusing for users who want to install both variants,
so fix it by using different registry keys.
Signed-off-by: NStefan Weil <sw@weilnetz.de>

qemu.nsi

@@ -60,7 +60,11 @@ InstallDir $PROGRAMFILES\qemu
 
 ; Registry key to check for directory (so if you install again, it will
 ; overwrite the old one automatically)
-InstallDirRegKey HKLM "Software\qemu" "Install_Dir"
+!ifdef W64
+InstallDirRegKey HKLM "Software\qemu64" "Install_Dir"
+!else
+InstallDirRegKey HKLM "Software\qemu32" "Install_Dir"
+!endif
 
 ; Request administrator privileges for Windows Vista.
 RequestExecutionLevel admin