Problem found in /etc/xen/script/block - it check xenstore fod device duplicates. This leads to increased start time and timeouting waiting for device connection.

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Prior to this update, starting multiple para-virtualized guests in quick succession could cause several instances of a hot plug script to compete for the same lock file. Consequently, the connection may have timed out with the following error message written to standard error:

  Error: Device [id] (vbd) could not be connected. Hotplug scripts not working.

This update applies a patch that corrects the locking mechanism, so that such timeouts no longer occur.

An advisory has been issued which should help the problem
described in this bug report. This report is therefore being
closed with a resolution of ERRATA. For more information
on therefore solution and/or where to find the updated files,
please follow the link below. You may reopen this bug report
if the solution does not work for you.

http://rhn.redhat.com/errata/RHBA-2011-0326.html