x86/hvm: unify internal portio and mmio intercepts

x86/hvm: unify internal portio and mmio intercepts

The implementation of mmio and portio intercepts is unnecessarily different.
This leads to much code duplication. This patch unifies much of the
intercept handling, leaving only distinct handlers for stdvga mmio and dpci
portio. Subsequent patches will unify those handlers.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>