Our hotplug code supports macvtap insertion to guests. However, we
somehow forgot about 'attach-interface' (which tries to build XML from
passed arguments and use virDomainAttachDeviceFlags()).
New type is accessible under 'direct' type, to keep the same type as
used in domain XML.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>