Generalize -machine command line option

Generalize -machine command line option

-machine somehow suggests that it selects the machine, but it doesn't.
Fix that before this command is set in stone.

Actually, -machine should supersede -M and allow to introduce arbitrary
per-machine options to the command line. That will change the internal
realization again, but we will be able to keep the user interface
stable.

Tested-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>