trace: update docs with meson build information

trace: update docs with meson build information

The documentation still refers to the makefile and the old sub-directory
layout. Meson works differently: tracetool output is placed into the
builddir with mangled filenames like <builddir>/trace/trace-accel_kvm.h
for the accel/kvm/ trace.h definition.

This meson setup also requires a manually-created accel/kvm/trace.h file
that #includes the <builddir>/trace/trace-accel_kvm.h file. Document
this!

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20210112165859.225534-3-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>