sheep: remove invalid error logs produced by dog vdi setattr

This bug is reported in https://bugs.launchpad.net/sheepdog-project/+bug/1273080
To remove invalid error logs, I changed to skip sd_read_object() for existence check in sheep/vdi.c
if -x option is not specified to dog vdi setattr.

=== Bug Description ===
I got error logs when I added new attribute on vdi.
It seems that sheep searched for existing attr and couldn't find. Then logged error but created attr normaly.
I think these log should be INFO or WARN level.

Environment and versions
sheepdog : tag 0.7.6 (4 nodes)
operating system : CentOS 6.4

Here's the operations.

$ dog vdi list
  Name Id Size Used Shared Creation time VDI id Copies Tag
$ dog cluster format
using backend plain store
$ dog cluster format
using backend plain store
$ dog vdi create disk1 10m
$ dog vdi list
  Name Id Size Used Shared Creation time VDI id Copies Tag
  disk1 0 10 MB 0.0 MB 0.0 MB 2014-01-23 18:51 e8b18f 3
$ dog vdi setattr disk1 key1 value1
$ dog vdi getattr disk1 key1
value1

This returned 'value1' for key1 normally.

Here's the sheep.log

Jan 23 18:53:15 ERROR [gway 12101] sheep_exec_req(1062) failed No object found
Jan 23 18:53:15 ERROR [gway 12101] sheep_exec_req(1062) failed No object found
Jan 23 18:53:15 ERROR [gway 12101] sheep_exec_req(1062) failed No object found
Jan 23 18:53:15 ERROR [block] read_backend_object(414) failed to read object 20e8b18fc3b4ab67, No object found

No output on log when 'dog vdi getattr'.

Reviewed-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Reported-by: Yoshinori Matsuo <matsuo.yoshinori@lab.ntt.co.jp>
Signed-off-by: Yuichi Bando <bando.yuichi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>