Disallow colons in the parameter of "-accel"

Disallow colons in the parameter of "-accel"

Everybody who used something like "-machine accel=kvm:tcg" in the past
might be tempted to specify a similar list with the -accel parameter,
too, for example "-accel kvm:tcg". However, this is not how this
options is thought to be used, since each "-accel" should only take care
of one specific accelerator.

In the long run, we really should rework the "-accel" code completely,
so that it does not set "-machine accel=..." anymore internally, but
is completely independent from "-machine". For the short run, let's
make sure that users cannot use "-accel xyz:tcg", so that we avoid
that we have to deal with such cases in the wild later.

Message-Id: <20190930123505.11607-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>