virt-uname

# virt-uname CentOS5x32
Guest            System name      
CentOS5x32       Linux centos5x32.home.annexia.org 2.6.18-128.7.1.el5 #1 SMP Mon Aug 24 08:20:55 EDT 2009 i686 
# virt-uname CentOS5x32 --csv
Guest,System name
CentOS5x32,Linux centos5x32.home.annexia.org 2.6.18-128.7.1.el5 #1 SMP Mon Aug 24 08:20:55 EDT 2009 i686

The query is done over an authenticated and encrypted SNMPv3 connection into the guest. This requires that the guest is running snmpd, but the end game here would be to have snmpd installed routinely, with a minimal config that only answers localhost connections which are properly encrypted with the key known only by the host and the guest.

Code here, and previous discussion.

Edit:

# virt-uptime CentOS5x32
Guest            Uptime
CentOS5x32       3 hours, 16:33.96
# virt-ping 
Guest            Status           
CentOS5x32       ok