So that ejection of attached device by guest does not free data
in use by block migration instance.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
CC: Markus Armbruster <armbru@redhat.com>

Index: qemu/block-migration.c
===================================================================
--- qemu.orig/block-migration.c
+++ qemu/block-migration.c
@@ -19,6 +19,7 @@
 #include "monitor.h"
 #include "block-migration.h"
 #include "migration.h"
+#include "blockdev.h"
 #include <assert.h>
 
 #define BLOCK_SIZE (BDRV_SECTORS_PER_DIRTY_CHUNK << BDRV_SECTOR_BITS)
@@ -299,6 +300,7 @@ static void init_blk_migration_it(void *
         bmds->completed_sectors = 0;
         bmds->shared_base = block_mig_state.shared_base;
         alloc_aio_bitmap(bmds);
+        drive_get_ref(drive_get_by_blockdev(bs));
 
         block_mig_state.total_sector_sum += sectors;
 
@@ -532,6 +534,7 @@ static void blk_mig_cleanup(Monitor *mon
 
     while ((bmds = QSIMPLEQ_FIRST(&block_mig_state.bmds_list)) != NULL) {
         QSIMPLEQ_REMOVE_HEAD(&block_mig_state.bmds_list, entry);
+        drive_put_ref(drive_get_by_blockdev(bmds->bs));
         qemu_free(bmds->aio_bitmap);
         qemu_free(bmds);
     }