x86/guest: Introduce {get,set}_reg() infrastructure

x86/guest: Introduce {get,set}_reg() infrastructure

Various registers have per-guest-type or per-vendor locations or access
requirements.  To support their use from common code, provide accessors which
allow for per-guest-type behaviour.

For now, just infrastructure handling default cases and expectations.
Subsequent patches will start handling registers using this infrastructure.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: 88d3ff7ab15da277a85b39735797293fb541c718
master date: 2022-01-20 16:32:11 +0000