virt: acrn: Use vfs_poll() instead of f_op->poll()

Use a more advanced function vfs_poll() in acrn_irqfd_assign().
At the same time, modify the definition of events.

Signed-off-by: Yejune Deng <yejune.deng@gmail.com>
Signed-off-by: Shuo Liu <shuo.a.liu@intel.com>
Link: https://lore.kernel.org/r/20210221133306.33530-1-shuo.a.liu@intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>