kernel: mm: rename Z_KERNEL_VIRT_* to K_MEM_KERNEL_VIRT_*

Renames:
  Z_KERNEL_VIRT_START to K_MEM_KERNEL_VIRT_START
  Z_KERNEL_VIRT_SIZE to K_MEM_KERNEL_VIRT_SIZE
  Z_KERNEL_VIRT_END to K_MEM_KERNEL_VIRT_END

This is part of a series to move memory management related
stuff from Z_ namespace into its own namespace.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>