hw/char/virtio-serial: Do not expose the 'emergency-write' property

hw/char/virtio-serial: Do not expose the 'emergency-write' property

The VIRTIO_CONSOLE_F_EMERG_WRITE feature bit was only set
in the hw_compat_2_7[] array, via the 'emergency-write=off'
property. We removed all machines using that array, lets remove
that property. All instances have this feature bit set and
it can not be disabled. VirtIOSerial::host_features mask is
now unused, remove it.

Reviewed-by: Mark Cave-Ayland <mark.caveayland@nutanix.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20260108033051.777361-28-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>