A quick walkthrough for tracing kvm-qemu with UST:

git clone git://git.lttng.org/userspace-rcu.git
(install: see README)
git clone git://git.lttng.org/ust.git
(install: see README)

In my case, I enabled qemu_malloc, qemu_realloc, qemu_free,
qemu_memalign, qemu_vmalloc, qemu_vfree by editing the "trace-events"
file in the toplevel source directory (this file is qemu-specific, not
the usual interface UST provides):

# qemu-malloc.c
qemu_malloc(size_t size, void *ptr) "size %zu ptr %p"
qemu_realloc(void *ptr, size_t size, void *newptr) "ptr %p size %zu newptr %p"
qemu_free(void *ptr) "ptr %p"

# osdep.c
qemu_memalign(size_t alignment, size_t size, void *ptr) "alignment %zu size %zu ptr %p"
qemu_vmalloc(size_t size, void *ptr) "size %zu ptr %p"
qemu_vfree(void *ptr) "ptr %p"

./configure --enable-trace-backend=ust
make

ldd x86_64-softmmu/qemu-system-x86_64
...
libust.so.0 => /usr/local/lib/libust.so.0 (0x00007fa39fc74000)
(it's linked with libust)

usttrace x86_64-softmmu/qemu-system-x86_64
...
stop it (ctrl-c)

See resulting trace in:
$HOME/.usttraces/hostname-timestamp

UST 0.15 traces can be viewed with lttv: git clone git://git.lttng.org/lttv.git
(install: view README)

To view the trace (e.g.):

% lttv -m textDump -t % /home/compudj/.usttraces/ok-20110715144748823020811/32575_5629652727107472206
...
ust.qemu_free: 1552840.525993542 (/home/compudj/.usttraces/ok-20110715144748823020811/32575_5629652727107472206/ust_6), 0, 0, , , 0, 0x0, MODE_UNKNOWN { ptr = 0x1187680 }
ust.qemu_free: 1552840.525994061 (/home/compudj/.usttraces/ok-20110715144748823020811/32575_5629652727107472206/ust_6), 0, 0, , , 0, 0x0, MODE_UNKNOWN { ptr = 0x1187660 }