char: fix avail_connections init in qemu_chr_open_eventfd()

When trying to use a ivshmem server with qemu, ivshmem init code tries to
create a CharDriverState object for each eventfd retrieved from the server.
To create this object, a call to qemu_chr_open_eventfd() is done.
Right after this, before adding a frontend, qemu_chr_fe_claim_no_fail() is
called.
qemu_chr_open_eventfd() does not set avail_connections to 1, so no frontend can
be associated because qemu_chr_fe_claim_no_fail() makes qemu stop right away.

This problem comes from 456d606923
"qemu-char: Call fe_claim / fe_release when not using qdev chr properties".

Fix this, by setting avail_connections to 1 in qemu_chr_open_eventfd().

Signed-off-by: David Marchand <david.marchand@6wind.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

qemu-char.c

@@ -2493,7 +2493,13 @@ static gboolean tcp_chr_read(GIOChannel *chan, GIOCondition cond, void *opaque)
 #ifndef _WIN32
 CharDriverState *qemu_chr_open_eventfd(int eventfd)
 {
-    return qemu_chr_open_fd(eventfd, eventfd);
+    CharDriverState *chr = qemu_chr_open_fd(eventfd, eventfd);
+
+    if (chr) {
+        chr->avail_connections = 1;
+    }
+
+    return chr;
 }
 #endif