xemnstored: Fix xenstored abort when connection dropped.

xemnstored: Fix xenstored abort when connection dropped.

If a connection is dropped with pending input and output data then the
connection will be dereferenced by both handle_input and handle_output
resulting in a double free when the main loop dereferences the
connection.

Fix this issue by taking/releasing a reference over the calls to
handle_input and handle_output separately and checking the result of
talloc_free to see if the connection went away.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>