hostmem: set default prealloc_threads to valid value

Commit 4ebc74d removed default prealloc_threads initialization
by mistake, and that makes QEMU crash with division on zero at
  numpages_per_thread = numpages / memset_num_threads;
when QEMU is started with following backend
  -object memory-backend-ram,id=ram-node0,prealloc=yes,size=128M

Return back initialization removed by 4ebc74d to fix issue.

Fixes: 4ebc74d
Reported-by: Raphael Norwitz <[email protected]>
Signed-off-by: Igor Mammedov <[email protected]>
Reviewed-by: Laurent Vivier <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

Author: Igor Mammedov

backends/hostmem.c

@@ -280,6 +280,7 @@ static void host_memory_backend_init(Object *obj)
     /* TODO: convert access to globals to compat properties */
     backend->merge = machine_mem_merge(machine);
     backend->dump = machine_dump_guest_core(machine);
+    backend->prealloc_threads = 1;
 }
 
 static void host_memory_backend_post_init(Object *obj)