block: Add return value for bdrv_flush_all()

block: Add return value for bdrv_flush_all()

bdrv_flush() can fail, and bdrv_flush_all() should return an error as
well if this happens for a block device. It returns the first error
return now, but still at least tries to flush the remaining devices even
in error cases.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit f0f0fdfeec6c67ad374114ecc4b3e3ccde5e94d2)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>