Since we can easily lookup the vtable for a NMSetting8021xSchemeType,
it is convenient to also easily get the scheme_type for a given
NMSetting8021xSchemeVtable.

On my x86_64, this change is basically for free as it does not increase
the size of NMSetting8021xSchemeVtable, because the scheme_type fits in a
previously unused part of the NMSetting8021xSchemeVtable struct.