kvm tools: Improve "kvm setup" info and error messages

kvm tools: Improve "kvm setup" info and error messages

This patch improves the output of 'kvm setup' as follows:

  [penberg@tux kvm]$ ./kvm setup

  kvm setup creates a new rootfs under /home/penberg//.kvm-tools/.
  This can be used later by the '-d' parameter of 'kvm run'.

   usage: kvm setup [name]

  [penberg@tux kvm]$ ./kvm setup test-box
  A new rootfs 'test-box' has been created in '/home/penberg//.kvm-tools/test-box'.

  You can now start it by running the following command:

    kvm run -d test-box

  [penberg@tux kvm]$ ./kvm setup test-box
  Unable to create rootfs in /home/penberg//.kvm-tools/test-box: File exists

Acked-by: Cyrill Gorcunov <gorcunov@gmail.com>
Acked-by: Sasha Levin <levinsasha928@gmail.com>
Cc: Asias He <asias.hejun@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Pekka Enberg <penberg@kernel.org>