tools: probe for existence of qemu-xen trace backends.

tools: probe for existence of qemu-xen trace backends.

QEMU upstream commit ed7f5f1d8db0 ("trace: convert stderr backend to
log") renamed the stderr trace backend to log, which breaks the xen
build when pointed at a QEMU tree after that point:

./configure of QEMU fail with:
"ERROR: invalid trace backends
        Please choose supported trace backends."

These changes are not (yet) present in qemu-xen-unstable.git and in
any case we want to support QEMU before and after this change. Use the
tracetool.py provided by QEMU to probe for supported trace backends.

This is now done unconditionally (not depending on debug=y), which is
simpler to arrange here but also follows upstream QEMU which in
baf86d6b3ca0 ("trace: switch default backend to "log"") switched the
default from "nop" to "log", so we would have got log in debug=no
builds from then on anyway.

Tested with current qemu-xen-unstable (f165e581d9a6) and current QEMU
upstream master (f075c89f0a9c), the latter picked up via:
    QEMU_UPSTREAM_URL := /path/to/qemu-xen.git
which therefore tested the out of tree build aspect of this change.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Paul Durrant <paul.durrant@citrix.com>
Cc: Anthony PERARD <anthony.perard@citrix.com>