hw/9pfs: Use ioeventfd for 9p

hw/9pfs: Use ioeventfd for 9p

With ioeventfd:
[root@qemu-img-64 storage]# dd if=/dev/zero of=/storage/testx bs=8k count=131072 oflag=direct
131072+0 records in
131072+0 records out
1073741824 bytes (1.1 GB) copied, 26.767 s, 40.1 MB/s

Without:
[root@qemu-img-64 storage]# dd if=/dev/zero of=/storage/testx bs=8k count=131072 oflag=direct
131072+0 records in
131072+0 records out
1073741824 bytes (1.1 GB) copied, 65.3361 s, 16.4 MB/s

Reviewed-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Aneesh Kumar K.V <[email protected]>