Block migration fail, ignore error from bdrv_getlength

When there is no block driver associate with BlockDriverState bdrv_getlength
returns -ENOMEDIUM that cause block migration to fail

Signed-off-by: Shahar Havivi <shaharh@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/block-migration.c b/block-migration.c
index 7db6f02..a77106e 100644
--- a/block-migration.c
+++ b/block-migration.c
@@ -238,7 +238,7 @@
 
     if (!bdrv_is_read_only(bs)) {
         sectors = bdrv_getlength(bs) >> BDRV_SECTOR_BITS;
-        if (sectors == 0) {
+        if (sectors <= 0) {
             return;
         }