bhyve: add VNC password support

bhyve: add VNC password support

Support setting a password for the VNC framebuffer using the passwd
attribute on the <graphics/> element, if the driver has the
BHYVE_CAP_VNC_PASSWORD capability.

Note that virsh domxml-from-native does not output the password in the
generated XML, as VIR_DOMAIN_DEF_FORMAT_SECURE is not set when
formatting the domain definition.

Signed-off-by: Fabian Freyer <fabian.freyer@physik.tu-berlin.de>
Signed-off-by: Roman Bogorodskiy <bogorodskiy@gmail.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>