x86/shadow: ditch dangerous declarations

x86/shadow: ditch dangerous declarations

This started out with me noticing the latent bug of there being HVM
related declarations in common.c that their producer doesn't see, and
that hence could go out of sync at any time. However, go farther than
fixing just that and move the functions actually using these into hvm.c.
This way the items in question can simply become static, and no separate
declarations are needed at all.

Within the moved code constify and rename or outright delete the struct
vcpu * local variables and re-format a comment.

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