IBM is weird.  This seems to make it work on single cpu or multi cpu guests:

diff --git a/loader/linuxrc.s390 b/loader/linuxrc.s390
index 288dc12..498c68f 100644
--- a/loader/linuxrc.s390
+++ b/loader/linuxrc.s390
@@ -50,6 +50,7 @@ elif [ "${0##*/}" = "halt" ]; then
 fi
 
 VERSION=1.2
+NUMCPUS=$(grep processors /proc/cpuinfo | cut -c19-)
 
 export TEXTDOMAIN=s390installer
 export TEXTDOMAINDIR=/usr/lib/locale
@@ -150,7 +151,7 @@ function dasd_settle() {
         return 1
     fi
     local i=1
-    while [ $i -le 30 ] ; do
+    while [ $i -le 60 ] ; do
         local status
         read status < $dasd_status
         case $status in
@@ -225,14 +226,18 @@ function readcmsfile() # $1=dasdport $2=filename
     # precondition: dasd_eckd_mod driver incl. dependencies loaded,
     #               dasd_mod must be loaded without setting any DASD online
     dev=$(canonicalize_devno $1)
-    if ! sysecho /sys/bus/ccw/devices/$dev/online 1; then
-        echo $"DASD $dev could not be set online"
-        return 1
-    fi
-    udevadm settle
-    if ! dasd_settle $dev ; then
-        echo $"Could not access DASD $dev in time"
-        return 1
+    if [ ${NUMCPUS} -eq 1 ]; then
+        echo 1 > /sys/bus/ccw/devices/$dev/online
+    else
+        if ! sysecho /sys/bus/ccw/devices/$dev/online 1; then
+            echo $"DASD $dev could not be set online"
+            return 1
+        fi
+        udevadm settle
+        if ! dasd_settle $dev ; then
+            echo $"Could not access DASD $dev in time"
+            return 1
+        fi
     fi
     udevadm settle
     if ! cmsfscat -d /dev/dasda -a $2 > /tmp/$2; then

With anaconda-13.21.161-1.el6 and guest with one CPU, I'm getting this:

...
Initalizing network drop monitor service 
Freeing unused kernel memory: 260k freed 
grep: /proc/cpuinfo: No such file or directory 
 
Starting the zSeries initrd to configure networking. Version is 1.2
...
NFO kernel:TCP cubic registered 
INFO kernel:Initializing XFRM netlink socket 
INFO kernel:NET: Registered protocol family 17 
WARNING kernel:registered taskstats version 1 
INFO kernel:Initalizing network drop monitor service 
WARNING kernel:Freeing unused kernel memory: 260k freed 
/init: line 229: Ý: -eq: unary operator expected 
Could not access DASD 0.0.0191 in time 
/init: line 2762: /tmp/r6.conf: No such file or directory 
Scanning for available network devices... 
Autodetection found 0 devices. 
Note: There is a device blacklist active! (Clearing might take long) 
c) clear blacklist, m) manual config, r) rescan, s) shell: 


error/warning messages are also present on a guest with 2 CPUs, but CMS config file is read.
Moving to ASSIGNED

Created attachment 570343 [details]
proposed patch

Tested on a guests with 1 and 2 CPUs

    Technical note added. If any revisions are required, please edit the "Technical Notes" field
    accordingly. All revisions will be proofread by the Engineering Content Services team.
    
    New Contents:
Cause:
z/VM guests with only one CPU allocated fail to read the CMS configuration file used by the installation environment.

Consequence:
Users of z/VM guests with a single CPU must either pass all installation environment configuration values on the kernel boot line -or- supply the information at the interactive prompts as the installation environment boots up.

Fix:
Detect guests with one CPU and bring the boot device online so the CMS configuration file can be read using cmsfscat.

Result:
Users of z/VM guests with one CPU can provide a CMSCONF file on the kernel boot line to more easily automate installations.

Tested on RHEL6.3-20120416.0 (Beta), anaconda-13.21.167-1.el6.

No issues while installing on a guest with one or two CPUs, CMS config file was read, anaconda finished successfully.

Moving to VERIFIED.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

http://rhn.redhat.com/errata/RHBA-2012-0782.html