vl: extract various command line desugaring snippets to a new function
Keep the machine initialization sequence free of miscellaneous command
line parsing actions.
The only difference is that preallocation will always be done with one
thread if -smp is not provided; previously it was using mc->default_cpus,
which is almost always 1 anyway.
Reviewed-by: Igor Mammedov <imammedo@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>