libxl: support enabling and disabling <hap> feature
libxl: support enabling and disabling <hap> feature

Until now, the libxl driver ignored any <hap> setting in domain XML
and deferred to libxl, which enables hap if not specified. While
this is a good default, it prevents disabling hap if desired.

This change allows disabling hap with <hap state='off'/>. hap is
explicitly enabled with <hap/> or <hap state='on/>. Absense of <hap>
retains current behavior of deferring default state to libxl.

datto/libvirt,zippy2/libvirt,fabianfreyer/libvirt,libvirt/libvirt,fabianfreyer/libvirt,taget/libvirt,eskultety/libvirt,zippy2/libvirt,crobinso/libvirt,VenkatDatta/libvirt,crobinso/libvirt,jardasgit/libvirt,datto/libvirt,crobinso/libvirt,taget/libvirt,andreabolognani/libvirt,nertpinx/libvirt,zippy2/libvirt,olafhering/libvirt,nertpinx/libvirt,eskultety/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,VenkatDatta/libvirt,jfehlig/libvirt,libvirt/libvirt,zippy2/libvirt,nertpinx/libvirt,crobinso/libvirt,nertpinx/libvirt,taget/libvirt,libvirt/libvirt,fabianfreyer/libvirt,eskultety/libvirt,jardasgit/libvirt,datto/libvirt,jfehlig/libvirt,jfehlig/libvirt,olafhering/libvirt,taget/libvirt,taget/libvirt,olafhering/libvirt,andreabolognani/libvirt,jardasgit/libvirt,datto/libvirt,VenkatDatta/libvirt,olafhering/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,jardasgit/libvirt,datto/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,jfehlig/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,eskultety/libvirt,libvirt/libvirt,eskultety/libvirt