virnetsocket: Don't free virCommand in virNetSocketNewConnectCommand()

virnetsocket: Don't free virCommand in virNetSocketNewConnectCommand()

The aim of virNetSocketNewConnectCommand() is to execute passed
command and attach socket pair/pipe to it so that client socket
can be opened (this is used for connections with alternative
transports, e.g. ssh). The virCommand is created in a caller and
then passed to virNetSocketNewConnectCommand() where it is freed
using virCommandFree(). This approach is wrong on two levels:

1) The deallocation happens on a different level than allocation,
2) There's a WIN32 stub that just reports an error and doesn't
   free the command.

However, with g_autoptr() trickery the command can be freed in
caller.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>