frontend: 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.

spice has agent based file transfer support.
This patch adds support on the frontend to control those features
and allows to disable them whenever it is needed.

Change-Id: I99f9f2f878171746c613598d80fdc9fdd3208ed2
Bug-Url: https://bugzilla.redhat.com/1082479
Signed-off-by: Francesco Romani <fromani@redhat.com>