qemu: simplify monitor fd error handling

qemu: simplify monitor fd error handling

qemu_monitor was already returning -1 and setting errno to EINVAL
on any attempt to send an fd without a unix socket, but this was
a silent failure in the case of qemuDomainAttachHostPciDevice.
Meanwhile, qemuDomainAttachNetDevice was doing some sanity checking
for a better error message; it's better to consolidate that to a
central point in the API.

* src/qemu/qemu_hotplug.c (qemuDomainAttachNetDevice): Move sanity
checking...
* src/qemu/qemu_monitor.c (qemuMonitorSendFileHandle): ...into
central location.
Suggested by Chris Wright.