vdpa_sim: init iommu lock

The patch adding the iommu lock did not initialize it.
The struct is zero-initialized so this is mostly a problem
when using lockdep.

Reported-by: kernel test robot <rong.a.chen@intel.com>
Cc: Max Gurtovoy <maxg@mellanox.com>
Fixes: 0ea9ee43

 ("vdpasim: protect concurrent access to iommu iotlb")
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>