vdpa_sim_blk: set number of address spaces and virtqueue groups

Commit bda324fd ("vdpasim: control virtqueue support") added two
new fields (nas, ngroups) to vdpasim_dev_attr, but we forgot to
initialize them for vdpa_sim_blk.

When creating a new vdpa_sim_blk device this causes the kernel
to panic in this way:
    $ vdpa dev add mgmtdev vdpasim_blk name blk0
    BUG: kernel NULL pointer dereference, address: 0000000000000030
    ...
    RIP: 0010:vhost_iotlb_add_range_ctx+0x41/0x220 [vhost_iotlb]
    ...
    Call Trace:
     <TASK>
     vhost_iotlb_add_range+0x11/0x800 [vhost_iotlb]
     vdpasim_map_range+0x91/0xd0 [vdpa_sim]
     vdpasim_alloc_coherent+0x56/0x90 [vdpa_sim]
     ...

This happens because vdpasim->iommu[0] is not initialized when
dev_attr.nas is 0.

Let's fix this issue by initializing both (nas, ngroups) to 1 for
vdpa_sim_blk.

Fixes: bda324fd ("vdpasim: control virtqueue support")
Cc: gautam.dawar@xilinx.com
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20220621151323.190431-1-sgarzare@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Eugenio Pérez <eperezma@redhat.com>