Audit consistent option usage in documentation

qemu uses getopt_long_only(), which means that '-help' and '--help' are parsed identically; however, the short form is considered a crutch. Meanwhile, other binaries in the qemu package use getopt_long(), which requires the double-dash form. For consistency, all documentation examples should mention the long spelling with two dashes, especially since some options (like --object) have the same syntax between multiple binaries.

Note: This task needs to be discussed first on the mailing list. (Contact: @ebblake)