Minor bugfixes to improve the user experience

- Use genisoimage if mkisofs is not availiable.
- Be less prescriptive in xs_start_create_vm_with_cdrom. Don't abort if the vh
  folder exists already
- Use -o StrictHostKeyChecking=no

Signed-off-by: Robert Breker <robert.breker@citrix.com>
Merge pull request #7 from robertbreker/minorbugfixes

Minor bugfixes to improve the user experience