With hotplug, we're going to want to pass a tapfd name rather than an
actual file descriptor, so prepare the way by passing a string tapfd to
qemuBuildHostNetStr().

* src/qemu_conf.h: qemuBuildHostNetStr() takes a string tapfd now

* src/qemu_conf.c: pass qemuBuildHostNetStr() a string rather than an
  actual file descriptor

* src/qemu_driver.c: update qemudDomainAttachNetDevice() for change