vhost-vdpa: fix vm_flags for virtqueue doorbell mapping

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

vhost-vdpa: fix vm_flags for virtqueue doorbell mapping

The virtqueue doorbell is usually implemented via registeres but we don't provide the necessary vma->flags like VM_PFNMAP. This may cause several issues e.g when userspace tries to map the doorbell via vhost IOTLB, kernel may panic due to the page is not backed by page structure. This patch fixes this by setting the necessary vm_flags. With this patch, try to map doorbell via IOTLB will fail with bad address.