9p: use md5 of guest path as mount_tag

-mount_tag is limited 11 chars.

Signed-off-by: Hiroshi Miura <miurahr@linux.com>
mount_tag limit is 31 not 11

Signed-off-by: Hiroshi Miura <miurahr@linux.com>
Merge pull request #219 from miurahr/fix_p9share_index_issue203

9p: use md5 of guest path as mount_tag