scripts/kvm/kvm_stat: Cleanup of Groups class

scripts/kvm/kvm_stat: Cleanup of Groups class

Introduced separating newlines for readability and removed special
treatment/variable of the group leader. Renamed fmt to read_format.

The group leader's file descriptor will not be turned into a file
object anymore, instead os.read is used to read from the descriptor.

Signed-off-by: Janosch Frank <frankja@linux.vnet.ibm.com>
Message-Id: <1452525484-32309-24-git-send-email-frankja@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>