vl: make -accel help to list enabled accelerators only

vl: make -accel help to list enabled accelerators only

Currently, -accel help shows all possible accelerators regardless
if they are enabled in the binary or not. That is a different
semantic from -cpu and -machine helps, for example. So this change
makes it to list only the accelerators which support is compiled
in the binary target.

Note that it does not check if the accelerator is enabled in the
host, so the help message's header was rewritten to emphasize
that. Also qtest is not displayed given that it is used for
internal testing purpose only.

Signed-off-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-Id: <20190530215755.328-2-wainersm@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>