qemu-char: Add reconnecting to client sockets

Adds a "reconnect" option to socket backends that gives a reconnect
timeout.  This only applies to client sockets.  If the other end
of a socket closes the connection, qemu will attempt to reconnect
after the given number of seconds.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>