softmmu/vl.c: Handle '-cpu help' and '-device help' before 'no default machine'

softmmu/vl.c: Handle '-cpu help' and '-device help' before 'no default machine'

Currently if you try to ask for the list of CPUs for a target
architecture which does not specify a default machine type
you just get an error:

  $ qemu-system-arm -cpu help
  qemu-system-arm: No machine specified, and there is no default
  Use -machine help to list supported machines

Since the list of CPUs doesn't depend on the machine, this is
unnecessarily unhelpful. "-device help" has a similar problem.

Move the checks for "did the user ask for -cpu help or -device help"
up so they precede the select_machine() call which checks that the
user specified a valid machine type.

Signed-off-by: Peter Maydell <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>