win: Allow for selection specific image

This is useful if the install.wim contains multiple Windows versions
(e.g., Home, Pro, ..), because the autounattend file will always select
index 1. With this mechanism, a variant different from the first one can
be automatically selected. imageSelection can be either an index (1-N)
or the image name wiminfo can list all images contained in a given WIM
file.

The default case is index 1, which has the same effect as before, with
the possibility of having a slightly smaller install.wim file because
all unwanted variants are discarded.

wfvm/win.nix

@@ -5,6 +5,8 @@
 , impureMode ? false
 , installCommands ? []
 , users ? {}
+# autounattend always installs index 1, so this default is backward-compatible
+, imageSelection ? "1"
 , ...
 }@attrs:
 
@@ -106,10 +108,20 @@ let
         mkdir -p win/nix-win
         7z x -y ${windowsIso} -owin
 
-        # Split image so it fits in FAT32 partition
-        wimsplit win/sources/install.wim win/sources/install.swm 3072
+        # Extract desired variant from install.wim
+        # This is useful if the install.wim contains multiple Windows
+        # versions (e.g., Home, Pro, ..), because the autounattend file
+        # will always select index 1. With this mechanism, a variant different
+        # from the first one can be automatically selected.
+        # imageSelection can be either an index (1-N) or the image name
+        # wiminfo can list all images contained in a given WIM file
+        wimexport win/sources/install.wim "${imageSelection}" win/sources/install_selected.wim
         rm win/sources/install.wim
 
+        # Split image so it fits in FAT32 partition
+        wimsplit win/sources/install_selected.wim win/sources/install.swm 3072
+        rm win/sources/install_selected.wim
+
         cp ${autounattend.autounattendXML} win/autounattend.xml
 
         virt-make-fs --partition --type=fat win/ usbimage.img