block: Avoid bdrv_get_geometry() where errors should be detected bdrv_get_geometry() hides errors. Use bdrv_nb_sectors() or bdrv_getlength() instead where that's obviously inappropriate. Signed-off-by: Markus Armbruster <armbru@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Reviewed-by: Max Reitz <mreitz@redhat.com> Reviewed-by: Benoit Canet <benoit@irqsave.net> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>