qemu-options: Remove deprecated "-virtioconsole" option

It's been deprecated since QEMU 3.0, and nobody complained so far, so
it is time to remove this option now.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1544684731-18828-1-git-send-email-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>