libxl: support enabling the HPET

libxl accepts hpet configuration in its domain info struct.  Parse the
domain definition's <clock> element in order to set the value.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Apologies from Eric Blake, for mistakenly committing the broken
intermediate version.

src/libxl/libxl_conf.c

@@ -406,7 +406,7 @@ libxlMakeDomBuildInfo(virDomainDefPtr def, libxl_domain_config *d_config)
         for (i = 0; i < def->clock.ntimers; i++) {
             if (def->clock.timers[i]->name == VIR_DOMAIN_TIMER_NAME_HPET &&
                 def->clock.timers[i]->present == 1) {
-                b_info->u.hvm.acpi.hpet = 1;
+                b_info->u.hvm.hpet = 1;
             }
         }