qemu: retain default trace backend if 'ust' is not enabled

qemu: retain default trace backend if 'ust' is not enabled

The qemu build system enables the 'log' trace backend by default, if no
explicit choice was made with '--enable-trace-backend=CHOICE'.

However, the qemu recipe uses the following PACKAGECONFIG line:
    PACKAGECONFIG[ust] = "--enable-trace-backend=ust,--enable-trace-backend=nop,lttng-ust,"

which means that the 'nop' trace backend will be explicitly selected if the
'ust' feature is not enabled. The 'nop' backend removes almost all trace
points at compile time, and thus basically means 'disable tracing'.

To retain the default trace backend if 'ust' is not enabled, the above
PACKAGECONFIG line should either explicitly fall back to the 'log' backend,
or not provide any value for the 'disabled' case. This commit chooses the
latter to not make any assumption about the upstream default.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>