tcg: use QTree instead of GTree

tcg: use QTree instead of GTree

qemu-user can hang in a multi-threaded fork. One common
reason is that when creating a TB, between fork and exec
we manipulate a GTree whose memory allocator (GSlice) is
not fork-safe.

Although POSIX does not mandate it, the system's allocator
(e.g. tcmalloc, libc malloc) is probably fork-safe.

Fix some of these hangs by using QTree, which uses the system's
allocator regardless of the Glib version that we used at
configuration time.

Tested with the test program in the original bug report, i.e.:

Fixes: #285

Change-Id: I61bbe16ee5b639615a54f5afad0fa084c63c9b42
Signed-off-by: Emilio Cota <cota@braap.org>