qemu: support unmanaged macvtap devices with <interface type='ethernet'>

Traditionally, macvtap devices are supported using <interface
type='direct'>, but that type requires specifying a source device name
and macvtap mode which can't be altered after the initial device
creation (and may not even be available to the management software
that's creating the XML config to feed to libvirt).

But the attributes in the <source> are essentially describing how the
device will be connected to the network, and if libvirt is to be
supplied with the name of a macvtap device that has already been
created, that device will also already be connected to the network
(and the connection can't be changed). Thus it seems more appropriate
to use type='ethernet', which was created explicitly for this purpose
- for devices that have already been (or will be) connected to the
external network by someone/something outside of libvirt. The fact
that it is a *macv*tap rather than a contentional tap device is just a
detail.

This patch supports using an existing macvtap device with <interface
type='ethernet'> by checking the supplied target dev name to see if it
is a macvtap device and, when this is the case, calling
virNetDevMacVLanTapOpen() instead of virNetDevTapCreate(). For
consistency, this is only done when target managed='no'.

Resolves: https://bugzilla.redhat.com/1723367 (partially)

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>