multifd: Fix flush of zero copy page send request

Make IO channel flush call after the inflight request has been drained
in multifd thread, or else we may missed to flush the inflight request.

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

configs/devices/x86_64-softmmu/x86_64-quintela-devices.mak

@@ -0,0 +1,7 @@
+# Boards:
+#
+CONFIG_ISAPC=n
+CONFIG_I440FX=n
+CONFIG_Q35=n
+CONFIG_MICROVM=y
+

configs/devices/x86_64-softmmu/x86_64-quintela2-devices.mak

@@ -0,0 +1,6 @@
+# Boards:
+#
+CONFIG_ISAPC=y
+CONFIG_I440FX=y
+CONFIG_Q35=y
+CONFIG_MICROVM=y