virt: acrn: Use EPOLLIN instead of POLLIN

This fixes the following sparse warning:
"sparse warnings: (new ones prefixed by >>)"
>> drivers/virt/acrn/irqfd.c:163:13: sparse: sparse: restricted __poll_t
 degrades to integer

Fixes: dcf9625f

 ("virt: acrn: Use vfs_poll() instead of f_op->poll()")
Reported-by: kernel test robot <lkp@intel.com>
Acked-by: Shuo Liu <shuo.a.liu@intel.com>
Signed-off-by: Yejune Deng <yejune.deng@gmail.com>
Link: https://lore.kernel.org/r/20210310074901.7486-1-yejune.deng@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>