kvm: consistently return 0/-errno from kvm_convert_memory

In case of incorrect parameters, kvm_convert_memory() was returning
-1 instead of -EINVAL.  The guest won't notice because it will move
anyway to RUN_STATE_INTERNAL_ERROR, but fix this for consistency and
clarity.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

accel/kvm/kvm-all.c

@@ -2999,17 +2999,17 @@ int kvm_convert_memory(hwaddr start, hwaddr size, bool to_private)
     MemoryRegion *mr;
     RAMBlock *rb;
     void *addr;
-    int ret = -1;
+    int ret = -EINVAL;
 
     trace_kvm_convert_memory(start, size, to_private ? "shared_to_private" : "private_to_shared");
 
     if (!QEMU_PTR_IS_ALIGNED(start, qemu_real_host_page_size()) ||
         !QEMU_PTR_IS_ALIGNED(size, qemu_real_host_page_size())) {
-        return -1;
+        return ret;
     }
 
     if (!size) {
-        return -1;
+        return ret;
     }
 
     section = memory_region_find(get_system_memory(), start, size);
@@ -3027,7 +3027,7 @@ int kvm_convert_memory(hwaddr start, hwaddr size, bool to_private)
         if (!to_private) {
             return 0;
         }
-        return -1;
+        return ret;
     }
 
     if (!memory_region_has_guest_memfd(mr)) {