hostmem: default the amount of prealloc-threads to smp-cpus

Prior to the introduction of the prealloc-threads property, the amount
of threads used to preallocate memory was derived from the value of
smp-cpus passed to qemu, the amount of physical cpus of the host
and a hardcoded maximum value. When the prealloc-threads property
was introduced, it included a default of 1 in backends/hostmem.c and
a default of smp-cpus using the sugar API for the property itself. The
latter default is not used when the property is not specified on qemu's
command line, so guests that were not adjusted for this change suddenly
started to use the default of 1 thread to preallocate memory, which
resulted in observable slowdowns in guest boots for guests with large
memory (e.g. when using libvirt <8.2.0 or managing guests manually).

This commit restores the original behavior for these cases while not
impacting guests started with the prealloc-threads property in any way.

Fixes: 220c1fd864e9d ("hostmem: introduce "prealloc-threads" property")
Signed-off-by: Jaroslav Jindrak <dzejrou@gmail.com>
Message-Id: <20220517123858.7933-1-dzejrou@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

backends/hostmem.c

@@ -274,7 +274,7 @@ static void host_memory_backend_init(Object *obj)
     backend->merge = machine_mem_merge(machine);
     backend->dump = machine_dump_guest_core(machine);
     backend->reserve = true;
-    backend->prealloc_threads = 1;
+    backend->prealloc_threads = machine->smp.cpus;
 }
 
 static void host_memory_backend_post_init(Object *obj)