block: get max_transfer limit for char (scsi-generic) devices

We can get the maximum number of bytes for a single I/O transfer
from the BLKSECTGET ioctl, but we only perform this for block
devices.  scsi-generic devices are represented as character devices,
and so do not issue this today.  Update this, so that virtio-scsi
devices using the scsi-generic interface can return the same data.
Signed-off-by: NEric Farman <farman@linux.vnet.ibm.com>
Message-Id: <20170120162527.66075-4-farman@linux.vnet.ibm.com>
Reviewed-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

block/file-posix.c

@@ -674,7 +674,7 @@ static void raw_refresh_limits(BlockDriverState *bs, Error **errp)
     struct stat st;
 
     if (!fstat(s->fd, &st)) {
-        if (S_ISBLK(st.st_mode)) {
+        if (S_ISBLK(st.st_mode) || S_ISCHR(st.st_mode)) {
             int ret = hdev_get_max_transfer_length(bs, s->fd);
             if (ret > 0 && ret <= BDRV_REQUEST_MAX_BYTES) {
                 bs->bl.max_transfer = pow2floor(ret);