vnc: rework VncState release workflow.

Split socket closing and releasing of VncState into two steps.  First
close the socket and set the variable to -1 to indicate shutdown in
progress.  Do the actual release in a few places where we can be sure it
doesn't cause trouble in form of use-after-free.  Add some checks for a
valid socket handle to make sure we don't try to use the closed socket.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
