Add HAP to virDomainFeature enum

Add HAP to virDomainFeature enum

Extend the virDomainFeature enumeration to include HAP (hardware
assisted paging) feature.

Hardware features such as Extended Page Table and Nested Page
Table augment hypervisor software techniques such as shadow
page table.  Adding HAP to the virDomainFeature enumeration
allows users to select between hardware and software memory
management mechanisms for their guests.