OVMF: separate output for ovmf binaries

OVMF{,CODE,VARS}.fd are now available in a dedicated fd output, greatly
reducing the closure in the common case where only those files are used (a
few MBs versus several hundred MBs for the full OVMF).

Note: it's unclear why `dontPatchELF` is now necessary for the build to
pass (on my end, at any rate) but it doesn't make much sense to run this
fixup anyway,

Note: my reading of xen's INSTALL suggests that --with-system-ovmf should
point directly to the OVMF binary.  As such, the previous invocation was
incorrect (it pointed to the root of the OVMF tree).  In any case, I have
only built xen with `--with-system-ovmf`, I have not tested it.

Fixes https://github.com/NixOS/nixpkgs/issues/25854
Closes https://github.com/NixOS/nixpkgs/pull/25855

nixos/modules/virtualisation/libvirtd.nix

@@ -15,7 +15,7 @@ let
   '';
   qemuConfigFile = pkgs.writeText "qemu.conf" ''
     ${optionalString cfg.qemuOvmf ''
-      nvram = ["${pkgs.OVMF}/FV/OVMF_CODE.fd:${pkgs.OVMF}/FV/OVMF_VARS.fd"]
+      nvram = ["${pkgs.OVMF.fd}/FV/OVMF_CODE.fd:${pkgs.OVMF.fd}/FV/OVMF_VARS.fd"]
     ''}
     ${cfg.qemuVerbatimConfig}
   '';

nixos/modules/virtualisation/qemu-vm.nix

@@ -126,7 +126,7 @@ let
               bootFlash=$out/bios.bin
               ${qemu}/bin/qemu-img create -f qcow2 $diskImage "40M"
               ${if cfg.useEFIBoot then ''
-                cp ${pkgs.OVMF-CSM}/FV/OVMF.fd $bootFlash
+                cp ${pkgs.OVMF-CSM.fd}/FV/OVMF.fd $bootFlash
                 chmod 0644 $bootFlash
               '' else ''
               ''}