vmm/shell: Set DISK to image file path and print its value at startup
vmm/shell: Set DISK to image file path and print its value at startup

- Set the DISK environment variable to the absolute path of the VFS
disk image file

- Print the value of $DISK at startup

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr