Description of problem:

Like Julia's comments(https://bugzilla.redhat.com/show_bug.cgi?id=1716352#c30), 'fd' type error caused this issue. 

qemu-kvm process core dump directly when hotplugging tap deivce via qmp or hmp

Version-Release number of selected component (if applicable):

qemu-kvm: qemu-kvm-core-4.2.0-15.module+el8.2.0+6029+618ef2ec.x86_64
kernel: kernel-4.18.0-187.el8.x86_64

How reproducible:
always

Steps to Reproduce:
1. Boot qemu-kvm process 
/usr/libexec/qemu-kvm -qmp tcp:0:4444,server,nowait

2. qmp connect
# telnet localhost 4444

3. hotplug tap device
{"execute": "netdev_add", "arguments": {"type": "tap", "id": "idR6cfzQ", "fd": "50"}, "id": "yqUkNhhe"}


Actual results:

qemu-kvm: util/oslib-posix.c:247: qemu_set_nonblock: Assertion `f != -1' failed.
Aborted (core dumped)

Expected results:
works

Additional info: