Right now --with-git-submodules=ignore has a subtle difference from
just running without a .git directory, in that it does not check
that submodule sources actually exist.  Move the check for
ui/keycodemapdb/README so that it happens even if the user
specified --with-git-submodules=ignore, with a customized
error message that is more suitable for this situation.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>