ISCSI: redo how we set up the events

Call qemu_notify_event() after updating events.  Otherwise, If we add
an event for -is-writeable but the socket is already writeable there
may be a delay before the event callback is actually triggered.

Those delays would in particular hurt performance during BIOS boot and
when the GRUB bootloader reads the kernel and initrd.

But first call out to the socket write functions directly, and only set up
the write event if the socket is full.  This will happen very rarely and
this improves performance.
Signed-off-by: NRonnie Sahlberg <ronniesahlberg@gmail.com>

block/iscsi.c

@@ -39,6 +39,7 @@ typedef struct IscsiLun {
     int lun;
     int block_size;
     unsigned long num_blocks;
+    int events;
 } IscsiLun;
 
 typedef struct IscsiAIOCB {
@@ -104,11 +105,27 @@ static void
 iscsi_set_events(IscsiLun *iscsilun)
 {
     struct iscsi_context *iscsi = iscsilun->iscsi;
+    int ev;
 
-    qemu_aio_set_fd_handler(iscsi_get_fd(iscsi), iscsi_process_read,
-                           (iscsi_which_events(iscsi) & POLLOUT)
-                           ? iscsi_process_write : NULL,
-                           iscsi_process_flush, iscsilun);
+    /* We always register a read handler.  */
+    ev = POLLIN;
+    ev |= iscsi_which_events(iscsi);
+    if (ev != iscsilun->events) {
+        qemu_aio_set_fd_handler(iscsi_get_fd(iscsi),
+                      iscsi_process_read,
+                      (ev & POLLOUT) ? iscsi_process_write : NULL,
+                      iscsi_process_flush,
+                      iscsilun);
+
+    }
+
+    /* If we just added an event, the callback might be delayed
+     * unless we call qemu_notify_event().
+     */
+    if (ev & ~iscsilun->events) {
+        qemu_notify_event();
+    }
+    iscsilun->events = ev;
 }
 
 static void