python-blivet: Fix pvscan to not require lvmetad.

The --cache argument to pvscan won't work on NixOS, for specifics about this,
please have a look at:

http://www.mail-archive.com/pld-cvs-commit@lists.pld-linux.org/msg310821.html

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

pkgs/top-level/python-packages.nix

@@ -473,6 +473,7 @@ pythonPackages = python.modules // rec {
       }' blivet/formats/__init__.py
       sed -i -e 's|"lsof"|"${pkgs.lsof}/bin/lsof"|' blivet/formats/fs.py
       sed -i -r -e 's|"(u?mount)"|"${pkgs.utillinux}/bin/\1"|' blivet/util.py
+      sed -i '/pvscan/s/, *"--cache"//' blivet/devicelibs/lvm.py
     '';
 
     propagatedBuildInputs = let