qemu-char: (Re-)connect for tcp_chr_write() unconnected writing

qemu-char: (Re-)connect for tcp_chr_write() unconnected writing

tcp_chr_write() did not deal with writing to an unconnected
connection and return the original length of the data, it's
not right and would cause false writing. So (re-)connect it
and return 0 for this situation.

Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Lei Li <lilei@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>