Renames:
Z_VIRT_RAM_START to K_MEM_VIRT_RAM_START
Z_VIRT_RAM_SIZE to K_MEM_VIRT_RAM_SIZE
Z_VIRT_RAM_END to K_MEM_VIRT_RAM_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>