As we change bdrv_is_allocated to gather more information from bs and
bs->file, it will become a bit slower.  It is still appropriate for online
jobs, but not for reads/writes.  Call the internal function instead.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>