conf: make virDomainNetGetActualVlan arg/return val const

conf: make virDomainNetGetActualVlan arg/return val const

This is needed if we want to call the function when the
virDomainNetDef* we have is a const.

Since virDomainNetGetActualVlan returns a pointer to memory that is
within the virDomainNetDefPtr arg, the returned pointer must also be
made const. This leads to a cascade of other virNetDevVlanPtr's that
must be changed to "const virNetDevVlan *".

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