x86: don't offer Hyper-V option when "PV Shim Exclusive"

x86: don't offer Hyper-V option when "PV Shim Exclusive"

This only added dead code. Use "if" instead of "depends on" to make
(halfway) clear that other guest options should also go in the same
block. Move the option down such that the shim related options get
presented first, avoiding to ask the question when the answer may end
up being discarded.

While in the neighborhood also bring PV_SHIM_EXCLUSIVE into more
"canonical" shape.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wl@xen.org>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>