net/qemu: move vlan/bandwidth validation out of network driver

In the past the network driver was (mistakenly) being called for all
interfaces, not just those of type='network', and so it had a chance
to validate all interface configs after the actual type of the
interface was known.

But since the network driver has been more completely/properly
separated from qemu, the network driver isn't called during the
startup of any interfaces except those with type='network', so this
validation no longer takes place for, e.g. <interface type='bridge'>
(or direct, etc). This in turn meant that a config could erroneously
specify a vlan tag, or bandwidth settings, for a type of interface
that didn't support it, and the domain would start without complaint,
just silently ignoring those settings.

This patch moves those validation checks out of the network driver,
and into virDomainActualNetDefValidate() so they will be done for all
interfaces, not just type='network'.

https://bugzilla.redhat.com/1741121
Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Cole Robinson <crobinso@redhat.com>