property strings: introduce key grouping feature

property strings: introduce key grouping feature

Use case: networks for kvm use a <model>=<macaddr> scheme
where the model represents the network card. The schema
previously could not represent this, so we now introduce a
'group' key which works similar to an alias with the
difference that the data structure also gets an entry named
after the group filled with the name of the key that was
used to fill it.

Usage:
{
    virtio => { group => 'model' },
    e1000 => { group => 'model' },
    model => {
        type => 'string',
        pattern => ... # pattern for mac address
        ...
    }
}

Now the string 'virtio=aa:bb:cc:dd:ee:ff' gets parsed into:
{
    model => 'virtio',
    virtio => 'aa:bb:cc:dd:ee:ff'
}

Error examples:
  With bad value:
    virtio: value does not match the regex pattern
  Missing group:
    model: property is missing and it is not optional

parse_net() however used the 'macaddr' key for the mac
address, which can be achieved by aliasing 'model' to
'macaddr':
{
    virtio => { group => 'model' },
    e1000 => { group => 'model' },
    model => { alias => 'macaddr' },
    macaddr => {
        type => 'string',
        pattern => ... # pattern for mac address
        ...
    }
}

Then the above string will be parsed into:
{
    model => 'virtio',
    macaddr => 'aa:bb:cc:dd:ee:ff'
}

The error output now always shows the 'macaddr' key:
Error examples:
  With bad value:
    macaddr: value does not match the regex pattern
  Missing group:
    macaddr: property is missing and it is not optional

In order to support specifying no mac address we can now set
model.default_key = 1 and macaddr.optional = 1.
That way `virtio,bridge=vmbr2` gets parsed correctly into
just a model with no macaddr. This works because default
keys as aliases have previously not been supported and would
not have been aliased accordingly. This case is now also
taken into account when printing default keys, which is now
skipped if it is also an alias.