backend: spice: add 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 adds support on the backend to control those features
and allows to disable them whenever it is needed.
Default is enabled for backward compatibility.

Change-Id: I945e6b23df2b9d0f7c18a4f1e73c64e12e9e1a78
Bug-Url: https://bugzilla.redhat.com/1082479
Signed-off-by: Francesco Romani <fromani@redhat.com>