x86: skip further initialization for idle domains

x86: skip further initialization for idle domains

While in the end not really found necessary, early versions of the
patches to follow pointed out that we needlessly set up paging for idle
domains. Arranging for that to be skipped made me notice that we can at
once skip vMCE setup for them. Leverage to adjustment to further
re-arrange the way FPU setup gets skipped.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>