This is to fix the VM lockup problem on HT processors that occured after S3 resume (see ticket #52). The qvm-run command now takes additional two switches: --pause --unpause
This is really a workaround, until Xen implements proper suspend/resume mechanism for notfying DomUs about system-wide S3 sleep. See this thread for more details: http://lists.xensource.com/archives/html/xen-devel/2010-07/msg00037.html