throw an error intead of returning Nothing when a reading the Qemu Configuration file failed
implement get method:
  I propose to use the following format to get something from the configuration
  file: "./tvm get <vmName> <field>" where field represent the explicite path
  to access a field. For example:
  sh$ ./tvm get test1 qemuArch
  ArchX86_64
  42sh$ ./tvm get test1 qemuDrives.0.diskFile
  "{HOME}/.tvm/disk/test1.dsk"

  Because qemuDrives is a list we need to provide explicitely the index we are
  going to access. If the field requested is a list of something, the return
  value will be the length of this list:
  42sh$ ./tvm get test1 qemuDrives
  2

  But if user requests to a structure the output is the result of show:
  42sh$ ./tvm get test1 qemuDrives.1
  Disk {diskFile = "cd1.iso", diskInterface = Nothing, diskMedia = MediaCDROM}