xen: parse and generate hpet item in sxpr

xen: parse and generate hpet item in sxpr

Recent versions of Xen disable the virtual HPET by default.  This is
usually more precise because tick policies are not implemented for
the HPET in Xen.  However, there may be several reasons to control
the HPET manually: 1) to test the emulation; 2) because distros may
provide the knob while leaving the default to "enabled" for compatibility
reasons.

This patch provides support for the hpet item in both sexpr and xm
formats, and translates it to a <timer> element.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>