CA-150952: properly check for XAPI session existence

Don't check if a XAPI session exists by comparing session with the empty
string because if a XAPI session does exist it returns a error string,
which evaluates to true.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>

(cherry picked from commit 3fd6375cc13cb4106153b15c31c62580fd7dcc89)