block migration: propagate return value when bdrv_write() returns < 0

Currently block_load() doesn't check return value of bdrv_write(), and
even the destination weren't prepared to execute block migration, it
proceeds and guest boots on the target.  This patch fix this issue.

Signed-off-by: Yoshiaki Tamura <tamura.yoshiaki@lab.ntt.co.jp>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
