vhost: remove assertion to prevent crash

QEMU will assert on vhost-user backed virtio device hotplug if QEMU is
using more RAM regions than VHOST_MEMORY_MAX_NREGIONS (for example if
it were started with a lot of DIMM devices).

Fix it by returning error instead of asserting and let callers of
vhost_set_mem_table() handle error condition gracefully.

Cc: qemu-stable@nongnu.org
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Jay Zhou <jianjay.zhou@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
