accel: cleanup error output

Only emit "XXX accelerator not found", if there are not
further accelerators listed. eg

   accel=kvm:tcg

doesn't print a "KVM accelerator not found" warning
when it falls back to tcg, but a

   accel=kvm

prints a warning, since no fallback is given.

Suggested-by: Daniel P. Berrange <berrange@redhat.com>
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <20170717144527.24534-1-lvivier@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>