accel/kvm: Check ioctl(KVM_SET_USER_MEMORY_REGION) return value

kvm_vm_ioctl() can fail, check its return value, and log an error
when it failed. This fixes Coverity CID 1412229:

  Unchecked return value (CHECKED_RETURN)

  check_return: Calling kvm_vm_ioctl without checking return value

Reported-by: Coverity (CID 1412229)
Fixes: 235e8982ad ("support using KVM_MEM_READONLY flag for regions")
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Message-Id: <20200221163336.2362-1-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

accel/kvm/kvm-all.c

@@ -308,13 +308,23 @@ static int kvm_set_user_memory_region(KVMMemoryListener *kml, KVMSlot *slot, boo
         /* Set the slot size to 0 before setting the slot to the desired
          * value. This is needed based on KVM commit 75d61fbc. */
         mem.memory_size = 0;
-        kvm_vm_ioctl(s, KVM_SET_USER_MEMORY_REGION, &mem);
+        ret = kvm_vm_ioctl(s, KVM_SET_USER_MEMORY_REGION, &mem);
+        if (ret < 0) {
+            goto err;
+        }
     }
     mem.memory_size = slot->memory_size;
     ret = kvm_vm_ioctl(s, KVM_SET_USER_MEMORY_REGION, &mem);
     slot->old_flags = mem.flags;
+err:
     trace_kvm_set_user_memory(mem.slot, mem.flags, mem.guest_phys_addr,
                               mem.memory_size, mem.userspace_addr, ret);
+    if (ret < 0) {
+        error_report("%s: KVM_SET_USER_MEMORY_REGION failed, slot=%d,"
+                     " start=0x%" PRIx64 ", size=0x%" PRIx64 ": %s",
+                     __func__, mem.slot, slot->start_addr,
+                     (uint64_t)mem.memory_size, strerror(errno));
+    }
     return ret;
 }