qemu-char: fix qemu_chr_fe_get_msgfd()

qemu-char: fix qemu_chr_fe_get_msgfd()

Commit c76bf6bb8fbbb233a7d3641e09229d23747d5ee3 ("Add chardev API
qemu_chr_fe_get_msgfds") broke qemu_chr_fe_get_msgfd() because it
changed the return value.

Callers expect -1 if no fd is available.  The commit changed the return
value to 0 (which is a valid file descriptor number) so callers always
detected a file descriptor even if none was available.

This patch fixes qemu-iotests 045:

  $ cd tests/qemu-iotests && ./check 045
  [...]
  +FAIL: test_add_fd_invalid_fd (__main__.TestFdSets)
  +----------------------------------------------------------------------
  +Traceback (most recent call last):
  +  File "./045", line 123, in test_add_fd_invalid_fd
  +    self.assert_qmp(result, 'error/class', 'GenericError')
  +  File "/home/stefanha/qemu/tests/qemu-iotests/iotests.py", line 232, in assert_qmp
  +    result = self.dictpath(d, path)
  +  File "/home/stefanha/qemu/tests/qemu-iotests/iotests.py", line 211, in dictpath
  +    self.fail('failed path traversal for "%s" in "%s"' % (path, str(d)))
  +AssertionError: failed path traversal for "error/class" in "{u'return': {u'fdset-id': 2, u'fd': 0}}"

Cc: Nikolay Nikolaev <n.nikolaev@virtualopensystems.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>