hmp/vnc: Fix info vnc list leak

hmp/vnc: Fix info vnc list leak

We're iterating the list, and then freeing the iteration pointer rather
than the list head.

Fixes: 0a9667ecdb6d ("hmp: Update info vnc")
Reported-by: Coverity (CID 1421932)
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20200323120822.51266-1-dgilbert@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
(cherry picked from commit d4ff109373ce871928c7e9ef648973eba642b484)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>