Issue#536: raise proper error message when disk os info absent

When disk os cannot be decided by libguestfs,
it will leave it to 'unknown' distro and version.
But if no boot information available,
ImageFormatError is left unhandled and this will introduce
500 error when disk os info is absent.
Handle this error to fix this.

Signed-off-by: Royce Lv <lvroyce@linux.vnet.ibm.com>