[RHEL5.6] KVM: can't determine target name for virsh detach-disk when repeating attach and detach

Issue

• If a disk is online attached/detached to a RHEL5.x guest OS repeatedlyusing a same target name (e.g. vda), the target device name recognized by the guest OS is incremented (e.g. vda => vdb => vdc => ...).
• As a result, admins can't easily determine a target name to detach from the guest OS, since there is no target name mapping rule between host and guest.
• How reproducible: Always
• This problem occurs on RHEL5.5, too.  So not a regression on RHEL5.6.