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.