First make it build on libnl1/2.  Second, the VLAN
virtual interface name might not always be given in the
NMConnection (if the master is a UUID and thus the name
is determined automatically) so just take the interface
name instead.  And make sure we verify it's a VLAN
interface before deleting it.

Lastly, construct the VLAN interface name if it's not
given in the NMConnection.  This means we need to know
the master interface name when creating the connection,
which we always will since you can't create the VLAN
interface without it's master being present.  That also
means we need to return the name to the caller so it
can be used to create the NMDevice for the VLAN interface
after we've created it in the kernel.