Fix error output in LUSetClusterParams

Before:
  gnt-cluster modify --enabled-hypervisors=xen-hvm,xem-pvm
  Failure: command execution error:
  Enabled hypervisors contains invalid entries: set([u'xem-pvm'])

After:
  gnt-cluster modify --enabled-hypervisors=xen-hvm,xem-pvm
  Failure: command execution error:
  Enabled hypervisors contains invalid entries: 'xem-pvm'

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>