Add kernel/initrd/cmd_line directives for the qemu driver
Added entries in README.md
Add network debugging output
Remove one branch that can never be taken and comment another that can only be taken in older vagrant
Create default network if needed. Relates to #102.

This commit

* renames the option default_network to default_network_name
* introduces the option default_network_address, since we need an
address in order to create a network
* handles creation of the default network similarly to other private
networks if needed
Fail if any existing networks configured to have DHCP do not have it. Relates to #103 and #123.
Make network creation thread-safe, closes #104
Fix logic for debug output about finding network by ip
Use vagrant-libvirt and 192.168.121.1/24 as default network settings.
Closes #125.
Rename default network to management network. Closes #126.

The use of the term default makes discussing this network confusing.
This network must exist and be configured a certain way in order for vagrant
to manage a VMs configuration, so lets call it the management network instead.
Merge pull request #120 from teto/kernel2

Add kernel/initrd/cmd_line directives for the qemu driver
Merge branch 'improve_network_robustness' of github.com:sciurus/vagrant-libvirt into sciurus-improve_network_robustness

Conflicts:
	README.md
	lib/vagrant-libvirt/config.rb
remove not require return