remove useless package importation (also deprecated)
do not use deprecated function from Data.Aeson.Generic
Derive Qemu data to Generic and instance ToJSON and FromJSON class
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}
Merge pull request #1 from NicolasDP/master

Update Aeson to not use deprecated functions