x86/mm: put HVM only code under CONFIG_HVM

x86/mm: put HVM only code under CONFIG_HVM

Going through the code, HAP, EPT, PoD and ALTP2M depend on HVM code.
Put these components under CONFIG_HVM. This further requires putting
one of the vm event under CONFIG_HVM.

Altp2m requires a bit more attention because its code is embedded in
generic x86 p2m code.

Also make hap_enabled evaluate to false when !CONFIG_HVM. Make sure it
evaluate its parameter to avoid unused variable warnings in its users.

Also sort items in Makefile while at it.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Tamas K Lengyel <tamas@tklengyel.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>