tap: set vhostfd passed from qemu cli to non-blocking
    
    A guest boot hangs while probing the network interface when
    iommu_platform=on is used.
    
    The following qemu cli hangs without this patch:
    
    # $QEMU \
      -netdev tap,fd=3,id=hostnet0,vhost=on,vhostfd=4 3<>/dev/tap67 4<>/dev/host-net \
      -device virtio-net-pci,netdev=hostnet0,id=net0,iommu_platform=on,disable-legacy=on \
      ...
    
    Commit: c471ad0e9bd46 (vhost_net: device IOTLB support) took care of
    setting vhostfd to non-blocking when QEMU opens /dev/host-net but if
    the fd is passed from qemu cli then we need to ensure that fd is set
    to non-blocking.
    
    Fixes: c471ad0e9bd46 ("vhost_net: device IOTLB support")
    Cc: qemu-stable@nongnu.org
    Cc: Michael S. Tsirkin <mst@redhat.com>
    Cc: Jason Wang <jasowang@redhat.com>
    Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
    Signed-off-by: Jason Wang <jasowang@redhat.com>
    (cherry picked from commit d542800d1edc62f63f8a29cfa6bdd1a9536ae11c)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>