Document command line options with single dash

Document command line options with single dash

QEMU options have a single dash (but also work as double dash for
convenience and compatibility). Most options are listed with single
dash in command line help but some were listed with two dashes.
Normalize these to have the same format as the others.

Left --preconfig as that is mentioned as double dash everywhere so I
assume that is the preferred form for that.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Acked-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20180716193312.A5BA17456B9@zero.eik.bme.hu>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>