tools: probe for existence of qemu-xen stderr trace backend.

tools: probe for existence of qemu-xen stderr trace backend.

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."

Upstream also changed, in baf86d6b3ca0 ("trace: switch default backend
to "log""), to use "log" as the default backend (previously it was
"nop").

Use ./scripts/tracetool.py to check for the presence of the stderr
backend and if it is present then explicitly enable it. If the stderr
backend is not present then assume a newer QEMU which defaults to
"log" and simply accept that default (there is a 1 commit window
upstream where this would result in no trace backend being enabled).

The check is done using the older (deprecated?) --check-backend/--backend
variant of the tracetool.py options rather than the new plural
versions since the singular was supported even by very old versions of
QEMU.  New QEMU has compatibility code but if/when that is removed we
will still do the right thing i.e. no explict configuiration resulting
in the upstream default (currently "log").

If the explicit selection of the "stderr" backend is required then it
is now done unconditionally (not depending on debug=y), which is
simpler to arrange here but also matches the newer upstream's default
to "log" which is not conditional on debug being enabled either.

Tested with current qemu-xen-unstable (e9d8252) and current QEMU
upstream master (88c73d1), both out of tree via
QEMU_UPSTREAM_URL=/path/to/qemu-xen.git.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Paul Durrant <paul.durrant@citrix.com>
Cc: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>