Here is a proof of concept implementation for modern Linux guests (eg tested it on Fedora 21):

http://lists.nongnu.org/archive/html/qemu-devel/2014-12/msg02131.html

I've not checked whether it would work for older Linux (RHEL-6), and it also needs someone with Windows knowledge to provide the Win32 impl.

I have tried the command implemented by
commit 215a2771a7b6b29037ee8deba484815d816b6fdd
    qga: add guest-set-user-password command

on Fedora 22 Beta (qemu-guest-agent-2.3.0-0.2.rc1.fc22.x86_64)

And it fails with SELinux enforcing:
type=AVC msg=audit(1431952168.903:567): avc:  denied  { write } for  pid=2097 comm="chpasswd" name=".pwd.lock" dev="vda1" ino=33595649 scontext=system_u:system_r:virt_qemu_ga_t:s0 tcontext=system_u:object_r:passwd_file_t:s0 tclass=file permissive=0

Is it even possible to make this function work with SELinux?

Every new function added to qemu guest agent typically needs an addition to the SElinux policy to allow it, so not surprising that you see an AVC currently.

We have it upstream. Pending tasks:

 - Backport to 7.2
 - Create the SELinux Policy (we need a new BZ)

Libvirt support is already there.

I am looking at backport for 7.2

qemu-guest-agent has been rebased to 2.3 in 7.2, see dep bug 1194152

Test this bug using the following version:
Host:
kernel-3.10.0-302.el7.x86_64
qemu-kvm-rhev-2.3.0-15.el7.x86_64

RHEL7.2 guest:
kernel-3.10.0-302.el7.x86_64
qemu-guest-agent-2.3.0-2.el7.x86_64

1. boot guest:
 /usr/libexec/qemu-kvm -cpu SandyBridge -machine pc-i440fx-rhel7.2.0 -enable-kvm -m 4096 -smp 2,sockets=1,cores=2,threads=1 -name rhel7base  -drive file=/home/rhel7.2.qcow2,if=none,id=drive-virtio-disk0,format=qcow2,werror=stop,rerror=stop,aio=native,cache.direct=on -device virtio-blk-pci,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1 -boot menu=on -monitor stdio -netdev tap,id=hostnet0,vhost=on,script=/etc/ovs-ifup,downscript=/etc/ovs-downscript -device virtio-net-pci,netdev=hostnet0,mac=52:54:00:c6:4b:15,id=test -nodefaults -nodefconfig -spice port=5910,seamless-migration=on,disable-ticketing -vga qxl -global qxl-vga.vram_size=67108864 -global PIIX4_PM.disable_s3=0 -global PIIX4_PM.disable_s4=0 -device virtio-serial-pci,id=virtio-serial0,bus=pci.0,addr=0x9 -chardev socket,path=/tmp/qga.sock,server,nowait,id=qga0 -device virtserialport,bus=virtio-serial0.0,chardev=qga0,id=org.qemu.guest_agent.0,name=org.qemu.guest_agent.0 -qmp tcp:0:6666,server,nowait

2. start qemu-guest-agent service inside guest
# service qemu-guest-agent start

3. change the root password with "crypted":false
# echo -n "new" | base64
bmV3
# nc -U /tmp/qga.sock
{"execute":"guest-set-user-password","arguments":{"crypted":false,"username":"root","password":"bmV3"}}

4. change the root password with "crypted":true
#  openssl passwd -crypt newnew
DkJgpBjhzSbHM
# echo -n "DkJgpBjhzSbHM" | base64
RGtKZ3BCamh6U2JITQ==
# nc -U /tmp/qga.sock
{"execute":"guest-set-user-password","arguments":{"crypted":true,"username":"root","password":"RGtKZ3BCamh6U2JITQ=="}}

Actual result:
after step3, inside guest, check the root password is changed to "new"
after step4, inside guest, check the root password is changed to "newnew"

According to comment17, set this issue as 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.

https://rhn.redhat.com/errata/RHBA-2015-2217.html