runInLinuxVM, test-driver: use -cpu max instead of -cpu host

This appears to avoid requiring KVM when it’s not available. This is
what I originally though -cpu host did. Unfortunately not much
documentation available from the QEMU side on this, but this appears
to square with help:

$ qemu-system-x86 -cpu help
...
x86 host                  KVM processor with all supported host features
x86 max                   Enables all features supported by the accelerator in the current host
...

Whether we actually want to support this not clear, since this only
happens when your CPU doesn’t have full KVM support. Some Nix builders
are lying about kvm support though. Things aren’t too slow without it
though.

Fixes NixOS#85394

Alternative to NixOS#83920

nixos/lib/qemu-flags.nix

@@ -22,9 +22,9 @@ rec {
         else throw "Unknown QEMU serial device for system '${pkgs.stdenv.hostPlatform.system}'";
 
   qemuBinary = qemuPkg: {
-    x86_64-linux = "${qemuPkg}/bin/qemu-kvm -cpu host";
+    x86_64-linux = "${qemuPkg}/bin/qemu-kvm -cpu max";
     armv7l-linux = "${qemuPkg}/bin/qemu-system-arm -enable-kvm -machine virt -cpu host";
     aarch64-linux = "${qemuPkg}/bin/qemu-system-aarch64 -enable-kvm -machine virt,gic-version=host -cpu host";
-    x86_64-darwin = "${qemuPkg}/bin/qemu-kvm -cpu host";
+    x86_64-darwin = "${qemuPkg}/bin/qemu-kvm -cpu max";
   }.${pkgs.stdenv.hostPlatform.system} or "${qemuPkg}/bin/qemu-kvm";
 }

nixos/tests/installer.nix

@@ -74,7 +74,7 @@ let
       throw "Non-EFI boot methods are only supported on i686 / x86_64"
     else ''
       def assemble_qemu_flags():
-          flags = "-cpu host"
+          flags = "-cpu max"
           ${if system == "x86_64-linux"
             then ''flags += " -m 768"''
             else ''flags += " -m 512 -enable-kvm -machine virt,gic-version=host"''