vdpa_sim_blk: set number of address spaces and virtqueue groups

In the Linux kernel, the following vulnerability has been resolved:

vdpa_sim_blk: set number of address spaces and virtqueue groups

Commit bda324fd037a ("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.