bdrv_getlength could fail, check the return value before using it.
Return NULL and set errno if it fails. Callers are updated to handle
the error case.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>