VT-d: fix !HVM build

VT-d: fix !HVM build

EPT is of no interest when !HVM. While I'm observing gcc11 to fully
eliminate the function, older gcc's DCE looks to not be as good.
CONFIG_UBSAN apparently also limits how aggressively dead code is
eliminated. Aid the compiler in eliminating the accesses of
opt_hap_{2mb,1gb}, which otherwise cause undefined symbol errors when
linking.

While there adjust types.

Fixes: c479415610f0 ("x86/P2M: p2m.c is HVM-only")
Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>