util: make macvtap/macvlan generated name #defines available to other files
    
    MACVTAP_NAME_PREFIX and MACVLAN_NAME_PREFIX could be useful to other
    files if they were defined in virnetdevmacvlan.h instead of
    virnetdevmacvlan.c, so do that (while slightly renaming them and also
    adding yet another #define that chooses between macvlan/macvtap based
    on flags).
    
    This is a prerequisite to fix: https://bugzilla.redhat.com/1335798