kvm tools: Fix virtio console hangs by removing IRQ injection for tx path

kvm tools: Fix virtio console hangs by removing IRQ injection for tx path

As virtio spec says:

"""
 Because this is high importance and low bandwidth, the current Linux
 implementation polls for the buffer to be used, rather than waiting
 for an interrupt, simplifying the implementation signicantly.
"""

drivers/char/virtio_console.c
 send_buf() {
 ...
	/* Tell Host to go! */
	virtqueue_kick(out_vq);
 ...
        while (!virtqueue_get_buf(out_vq, &len))
                cpu_relax();
 ...
 }

The console hangs can simply be reproduced by yes command which
gives tremendous console IOs and IRQs.

[   16.786440] irq 4: nobody cared (try booting with the "irqpoll" option)
[   16.786440] Pid: 1437, comm: yes Tainted: G        W 2.6.39-rc6+ #56
[   16.786440] Call Trace:
[   16.786440]  [<c16578eb>] __report_bad_irq+0x30/0x89
[   16.786440]  [<c10980e6>] note_interrupt+0x118/0x17a
[   16.786440]  [<c1096e7d>] handle_irq_event_percpu+0x168/0x179
[   16.786440]  [<c1096eba>] handle_irq_event+0x2c/0x46
[   16.786440]  [<c1098516>] ? unmask_irq+0x1e/0x1e
[   16.786440]  [<c1098566>] handle_level_irq+0x50/0x6e
[   16.786440]  <IRQ>  [<c102fa69>] ? do_IRQ+0x35/0x7f
[   16.786440]  [<c1665ea9>] ? common_interrupt+0x29/0x30
[   16.786440]  [<c16610d6>] ? _raw_spin_unlock_irqrestore+0x7/0x28
[   16.786440]  [<c1364f65>] ? hvc_write+0x88/0x9e
[   16.786440]  [<c1355500>] ? do_output_char+0x88/0x18a
[   16.786440]  [<c1355631>] ? process_output+0x2f/0x42
[   16.786440]  [<c1355af6>] ? n_tty_write+0x211/0x2dc
[   16.786440]  [<c1059d77>] ? try_to_wake_up+0x226/0x226
[   16.786440]  [<c13534a4>] ? tty_write+0x15e/0x1d1
[   16.786440]  [<c12c1644>] ? security_file_permission+0x22/0x26
[   16.786440]  [<c13558e5>] ? process_echoes+0x241/0x241
[   16.786440]  [<c10dd9d2>] ? vfs_write+0x84/0xd7
[   16.786440]  [<c1353346>] ? tty_write_lock+0x3d/0x3d
[   16.786440]  [<c10ddb92>] ? sys_write+0x3b/0x5d
[   16.786440]  [<c166594c>] ? sysenter_do_call+0x12/0x22
[   16.786440] handlers:
[   16.786440] [<c1351397>] (vp_interrupt+0x0/0x3a)
[   16.786440] Disabling IRQ #4

Tested-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Asias He <asias.hejun@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>