Adjust non-default sized console ring allocation

Adjust non-default sized console ring allocation

Using xmalloc() for objects that are guaranteed to be at least as
large as a page is wasteful, as it will always result in more (here:
double the amount) being allocated.

The other adjustments are more cosmetic:
- Updating conring and conring_size can be done so NMI/MCE generated
  messages don't use the new (larger) size with the old (smaller)
  buffer.
- The size printed can be in KiB (for the value to be easier to grasp)
  since it is always a multiple of the default of 16KiB.

Signed-off-by: Jan Beulich <jbeulich@novell.com>