Files under src/util must not depend on src/conf
Solve the macvtap problem by moving the definition
of macvtap modes from domain_conf.h into macvtap.h

* src/util/macvtap.c, src/util/macvtap.h: Add enum
  for macvtap modes
* src/conf/domain_conf.c, src/conf/domain_conf.h: Remove
  enum for macvtap modes