add support for tap devices

- support tap devices
- use -netdev instead of -net
Merge pull request #61 from lnussel/master

remove installstep.pm, now in distro specific repos
networking documentation
fix codestyle
support virtio-scsi as hdd model

- when virtio-scsi is set as hdd model, create scsi controller and
set drives as scsi-hd
- this allows to have /dev/sd* devices in QEMU guest
Merge pull request #63 from aaannz/virtio-scsi_support

support virtio-scsi as hdd model
installstep is gone
Merge pull request #62 from aaannz/tap_support

support for TAP devices
write crash file in case of trouble in mgmt thread

also refactor code