kvm: add detail error message when fail to add ioeventfd

I try to hotplug 28 * 8 multiple-function devices to guest with
old host kernel, ioeventfds in host kernel will be exhausted, then
qemu fails to allocate ioeventfds for blk/nic devices.

It's better to add detail error here.
Signed-off-by: NAmos Kong <akong@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NGleb Natapov <gleb@redhat.com>

kvm-all.c

@@ -837,6 +837,8 @@ static void kvm_mem_ioeventfd_add(MemoryListener *listener,
                                data, true, int128_get64(section->size),
                                match_data);
     if (r < 0) {
+        fprintf(stderr, "%s: error adding ioeventfd: %s\n",
+                __func__, strerror(-r));
         abort();
     }
 }
@@ -869,6 +871,8 @@ static void kvm_io_ioeventfd_add(MemoryListener *listener,
                               data, true, int128_get64(section->size),
                               match_data);
     if (r < 0) {
+        fprintf(stderr, "%s: error adding ioeventfd: %s\n",
+                __func__, strerror(-r));
         abort();
     }
 }