pc: Implement -no-hpet as sugar for -machine hpet=on

Get rid of yet another global variable.

The default will be hpet=on only if CONFIG_HPET=y.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20201021144716.1536388-1-ehabkost@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>