restapi: spice: expose support to control the user data transfer

spice supports agent based file transfer support and clipboard
copy-paste. Those features are enabled by default, but can
be turned off independently.

This patch exposes the feature control toggle
through the REST API.

Change-Id: I337d0b46c40f5ffe20d153707c66a83acbb2a7c1
Bug-Url: https://bugzilla.redhat.com/1082479
Signed-off-by: Francesco Romani <fromani@redhat.com>