x86/p2m: {,un}map_mmio_regions() are HVM-only

x86/p2m: {,un}map_mmio_regions() are HVM-only

Mirror the "translated" check the functions do to do_domctl(), allowing
the calls to be DCEd by the compiler. Add ASSERT_UNREACHABLE() to the
original checks.

Also arrange for {set,clear}_mmio_p2m_entry() and
{set,clear}_identity_p2m_entry() to respectively live next to each
other, such that clear_mmio_p2m_entry() can also be covered by the
#ifdef already covering set_mmio_p2m_entry().

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>