provider/virtualbox: Refactor host-only network settings

[legal90]

I've refactored VagrantPlugins:: ProviderVirtualBox::Action:: Network#hostonly_config in order to make it more simpler, readable and safer to use:

• Simplified calculations of IP and DHCP settings. Handle IPs as IPAddr instances, not strings.
  Use IPAddr methods instead of Vagrant::Util::NetforkIP#network_address and string splitting.
• Added a basic validation of IP settings. A human-readable exception will be raised if the specified IP is invalid or netmask could not be used with that IP family. Currently, it is done only for private_network (aka host-only), but the validation could be moved to the global scope if you think it's reasonable. Examples:

# Vagranttile
config.vm.network "private_network", ip: "invalid-ip", netmask: '255.255.0.0'


# `vagrant up` output:
Network settings specified in your Vagrantfile are invalid:

Network settings: {:auto_config=>true, :mac=>nil, :nic_type=>nil, :type=>:static, :ip=>"invalid-ip", :netmask=>"255.255.0.0", :protocol=>"tcp", :id=>"d05cca65-6cb5-46f1-ba07-c891923e3219"}
Error: invalid address

# Vagranttile
config.vm.network "private_network", ip: "dead:beef::", netmask: '255.255.0.0'


# `vagrant up` output:
Network settings specified in your Vagrantfile are invalid:

Network settings: {:auto_config=>true, :mac=>nil, :nic_type=>nil, :type=>:static6, :ip=>"dead:beef::", :netmask=>"255.255.0.0", :protocol=>"tcp", :id=>"dddf6dea-0ee7-4afc-9d93-7dbd297661f6"}
Error: address family is not same

Existing unit tests for this class are passed. I've also verified it with manual tests on real VMs.

[legal90]

@chrisroberts Could you please review this?

[legal90]

I've rebased the branch and fixed a merge conflict.

[legal90]

Rebased

[chrisroberts]

Just updated the error output to be formatted instead of the raw hash. Otherwise, this looks great! Thanks so much and thanks for the rebases (and patience 😃) Cheers!

[ghost]

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