docs/config: Set the "kvm" accelerator via "[accel]" section

docs/config: Set the "kvm" accelerator via "[accel]" section

Configuring the accelerator should nowadays be done via the "-accel"
command line parameter, and thus via the "[accel]" section in config
files. We also need this change for the upcoming qtests that will
use these config files, since the qtests are already using "-accel"
for setting the "qtest" accelerator and QEMU does not like mixing
"-accel ..." and "-machine accel=...".

Message-Id: <20230228211533.201837-4-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>