xcl: Add PVH support

Giving support for PVH (#5) implies:

  * Enabling HAP;
  * Using native TSC mode;
  * Properly setting PVH features;
  * Not doing the phys to machine translation
  since it's an autotranslated guest;

Signed-off-by: Joao Martins <joao.martins@neclab.eu>
domain/xcl: Add PVH support when DOMLIB=xcl

Fixes #5

Signed-off-by: Joao Martins <joao.martins@neclab.eu>