x86/domain: push per-domain mapping creation down to hvm_domain_initialise

x86/domain: push per-domain mapping creation down to hvm_domain_initialise

We want to have a single entry point to initialise hvm guest.  Push
the per-domain mapping creation down to hvm_domain_initialise.

We can't move setting hap_enabled yet because that field needs to be
set before paging initialisation. Document that.

While at it, supply hvm_domain_initialise with more arguments. Though
they aren't used yet, they might be required in the future.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>