OVMF: get version number from edk2

OVMF is built from edk2 sources so that's where its version number comes
from (logically). The edk2 version number is 2014-12-10, so this change
only ensures the version numbers won't drift apart in the future. (There
is no hash change.)

pkgs/applications/virtualization/OVMF/default.nix

@@ -9,10 +9,11 @@ let
   else
     throw "Unsupported architecture";
 
+  version = (builtins.parseDrvName edk2.name).version;
 in
 
 stdenv.mkDerivation (edk2.setup "OvmfPkg/OvmfPkg${targetArch}.dsc" {
-  name = "OVMF-2014-12-10";
+  name = "OVMF-${version}";
 
   # TODO: properly include openssl for secureBoot
   buildInputs = [nasm iasl] ++ stdenv.lib.optionals (secureBoot == true) [ openssl ];