hpet: Report warnings with warn_report(), not error_printf()

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190417190641.26814-7-armbru@redhat.com>