xend: Fix xm block-detach regression introduced by c/s 15157.

xend: Fix xm block-detach regression introduced by c/s 15157.

Prior to this c/s, user was able to specify device name or id, e.g.
 xm block-detach dom xvdb
 xm block-detach dom 15728

Now, 'xm block-detach dom xvdb' silently fails.  xend.log does contain
the following

[2007-06-07 11:39:18 xend.XendDomainInfo 3775] DEBUG
(XendDomainInfo:519) devName=vbd/51712
[2007-06-07 11:39:18 xend.XendDomainInfo 3775] DEBUG
(XendDomainInfo:527) Could not find the device xvdb

This patch restores the previous behavior but retains the bugfix
introduced by c/s 15571.

Signed-off-by: Jim Fehlig <jfehlig@novell.com>