slirp: handle deferred ECONNREFUSED on non-blocking TCP sockets

slirp: handle deferred ECONNREFUSED on non-blocking TCP sockets

slirp currently only handles ECONNREFUSED in the case where connect()
returns immediately with that error; since we use non-blocking sockets,
most of the time we won't receive the error until we later try to read
from the socket.  Ensure that we deliver the appropriate RST to the
guest in this case.

Signed-off-by: Steven Luo <steven+qemu@steven676.net>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>