Per discussion with Don Dugger, it should be sufficient to enable AES
in qemu to allow this to be specifiied for Westmere beyond the Nehalem
definition.  I'm a little leery whether this was the entirety of the
customer request and need clarification before closing this off as
sufficient.  In any case:

diff --git a/target-i386/helper.c b/target-i386/helper.c
index 55aa7fb..f9ae62a 100644
--- a/target-i386/helper.c
+++ b/target-i386/helper.c
@@ -55,7 +55,7 @@ static const char *ext_feature_name[] = {
     NULL, "cx16", "xtpr", NULL,
     NULL, NULL, "dca", "sse4.1|sse4_1",
     "sse4.2|sse4_2", "x2apic", NULL, "popcnt",
-    NULL, NULL, NULL, NULL,
+    NULL, "aes", NULL, NULL,
     NULL, NULL, NULL, "hypervisor",
 };
 static const char *ext2_feature_name[] = {

Update to Comment #2:

After further review, Westmere should be broken out as a separate
model to allow family/stepping/model/level/etc.. to track the
actual silicon definitions.  Probably should be a rule of thumb
in similar cases:

http://post-office.corp.redhat.com/archives/rhvirt-patches/2010-November/msg00585.html

result:
guest boot successfully with -cpu Westmere,enforce on Westmere host, flag aes is exported to guest, guest family/model/stepping/level is the same as the info in dump

guest quit with error when boot with -cpu Westmere,enforce on host < Westmere

1. qemu-kvm & kernel
kernel: 2.6.32-113.el6.x86_64
qemu: qemu-kvm-0.12.1.2-2.144.el6.x86_64

2. cmd
qemu-kvm -monitor stdio -drive file=/images/RHEL-Server-6.0-32-virtio.qcow2,index=0,if=none,id=drive-virtio-disk1,media=disk,cache=none,format=qcow2,aio=native -device virtio-blk-pci,bus=pci.0,addr=0x4,drive=drive-virtio-disk1,id=virtio-disk1 -device virtio-net-pci,netdev=idx1i8qD,mac=9a:42:99:6d:c8:2d,netdev=idx1i8qD,id=ndev00idx1i8qD,bus=pci.0,addr=0x3 -netdev tap,id=idx1i8qD,script='/kvm/scripts/qemu-ifup-switch',downscript='no' -m 2048 -smp 2,cores=1,threads=1,sockets=2 -cpu Westmere,enforce -vnc :0 -rtc base=utc,clock=host,driftfix=none -usbdevice tablet -no-kvm-pit-reinjection -enable-kvm

3. host1 (E5620)

processor : 15
vendor_id : GenuineIntel
cpu family : 6
model : 44
model name : Intel(R) Xeon(R) CPU E5620 @ 2.40GHz
stepping : 2
cpu MHz : 2393.932
cache size : 12288 KB
physical id : 0
siblings : 8
core id : 10
cpu cores : 4
apicid : 21
initial apicid : 21
fpu : yes
fpu_exception : yes
cpuid level : 11
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 apic mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good xtopology nonstop_tsc aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 cx16 xtpr pdcm dca sse4_1 sse4_2 popcnt aes lahf_lm ida arat tpr_shadow vnmi flexpriority ept vpid

3. guest cpuinfo
processor : 1
vendor_id : GenuineIntel
cpu family : 6
model : 44
model name : Westmere E56xx/L56xx/X56xx (Nehalem-C)
stepping : 1
cpu MHz : 2393.932
cache size : 4096 KB
fpu : yes
fpu_exception : yes
cpuid level : 11
wp : yes
flags : fpu de pse tsc msr pae mce cx8 apic mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 syscall nx lm constant_tsc rep_good unfair_spinlock pni ssse3 cx16 sse4_1 sse4_2 x2apic popcnt aes hypervisor lahf_lm

4. guest quit with error when boot in 1216 host

Using CPU model "Westmere,enforce"
warning: host cpuid 0000_0001:ecx lacks requested flag 'ssse3' [0x00000200]
warning: host cpuid 0000_0001:ecx lacks requested flag 'sse4.1|sse4_1' [0x00080000]
warning: host cpuid 0000_0001:ecx lacks requested flag 'sse4.2|sse4_2' [0x00100000]
warning: host cpuid 0000_0001:ecx lacks requested flag 'popcnt' [0x00800000]
warning: host cpuid 0000_0001:ecx lacks requested flag 'aes' [0x02000000]
Unable to support requested x86 CPU definition

Add CCFR.

    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
    Intel cpu architecture "Westmere" not defined as a supported model.

Consequence
    Missing support for "aes" feature and local attributes for Westmere.

Change
    "Westmere" added as a supported Intel model, AES now recognized as
a cpu feature flag.

Result
    "Westmere" added as a supported Intel model, AES now recognized as
a cpu feature flag.

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/RHSA-2011-0534.html

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/RHSA-2011-0534.html