Replace '-machine accel=xyz' with '-accel xyz'

Replace '-machine accel=xyz' with '-accel xyz'

We've got a separate option to configure the accelerator nowadays, which
is shorter to type and the preferred way of specifying an accelerator.
Use it in the source and examples to show that it is the favored option.
(However, do not touch the places yet which also specify other machine
options or multiple accelerators - these are currently still better
handled with one single "-machine" statement instead)

Signed-off-by: Thomas Huth <thuth@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20190904052739.22123-1-thuth@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>