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