x86: properly use map_domain_page() in nested HVM code

x86: properly use map_domain_page() in nested HVM code

This eliminates a couple of incorrect/inconsistent uses of
map_domain_page() from VT-x code.

Note that this does _not_ add error handling where none was present
before, even though I think NULL returns from any of the mapping
operations touched here need to properly be handled. I just don't know
this code well enough to figure out what the right action in each case
would be.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>