qemu: simplify interface fd handling in monitor

qemu: simplify interface fd handling in monitor

With only a single caller to these two monitor commands, I
didn't need to wrap a new WithFds version, but just change
the command itself.

* src/qemu/qemu_monitor.h (qemuMonitorAddNetdev)
(qemuMonitorAddHostNetwork): Add parameters.
* src/qemu/qemu_monitor.c (qemuMonitorAddNetdev)
(qemuMonitorAddHostNetwork): Add support for fd passing.
* src/qemu/qemu_hotplug.c (qemuDomainAttachNetDevice): Use it to
simplify code.