Description of problem:
QEMU core dump when tap device with a non-exist fd.
qemu-kvm: util/oslib-posix.c:245: qemu_set_nonblock: Assertion `f != -1' failed.
ovmf.sh: line 30:   811 Aborted                 (core dumped) MALLOC_PERTURB_=1 /usr/libexec/qemu-kvm -S -name 'avocado-vt-vm1' -machine q35 -nodefaults -device VGA,bus=pcie.0,addr=0x1 -device pcie-root-port,id=pcie.0-root-port-3,slot=3,chassis=3,addr=0x3,bus=pcie.0 -device virtio-scsi-pci,id=virtio_scsi_pci0,bus=pcie.0-root-port-3,addr=0x0 -drive id=drive_image1,if=none,snapshot=off,aio=threads,cache=none,format=qcow2,file=/home/kvm_autotest_root/images/rhel810-64-virtio-scsi.qcow2 -device scsi-hd,id=image1,drive=drive_image1 -device pcie-root-port,id=pcie.0-root-port-4,slot=4,chassis=4,addr=0x4,bus=pcie.0 -device virtio-net-pci,mac=9a:91:92:93:94:95,id=id3cE67z,vectors=4,netdev=idR4I0Ft,bus=pcie.0-root-port-4,addr=0x0 -netdev tap,id=idR4I0Ft,vhost=on,vhostfd=22,fd=16 ....

(gdb) bt
#0  0x00007f3a81dcd93f in __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
#1  0x00007f3a81db7c95 in __GI_abort () at abort.c:79
#2  0x00007f3a81db7b69 in __assert_fail_base
    (fmt=0x7f3a81f1e860 "%s%s%s:%u: %s%sAssertion `%s' failed.\n%n", assertion=0x5560e23dd3bd "f != -1", file=0x5560e23dd34f "util/oslib-posix.c", line=245, function=<optimized out>) at assert.c:92
#3  0x00007f3a81dc5df6 in __GI___assert_fail
    (assertion=assertion@entry=0x5560e23dd3bd "f != -1", file=file@entry=0x5560e23dd34f "util/oslib-posix.c", line=line@entry=245, function=function@entry=0x5560e23dd550 <__PRETTY_FUNCTION__.20478> "qemu_set_nonblock") at assert.c:101
#4  0x00005560e223e4e1 in qemu_set_nonblock (fd=fd@entry=16) at util/oslib-posix.c:245
#5  0x00005560e2133f39 in net_init_tap (netdev=0x5560e43b3460, name=0x5560e43b3690 "idR4I0Ft", peer=0x0, errp=0x7fff7e7248d0) at net/tap.c:794
#6  0x00005560e2123c37 in net_client_init1 (errp=0x7fff7e7248d0, is_netdev=true, object=0x5560e43b3460) at net/net.c:1060
#7  0x00005560e2123c37 in net_client_init (opts=<optimized out>, is_netdev=<optimized out>, errp=0x7fff7e724bd0) at net/net.c:1154
#8  0x00005560e224d872 in qemu_opts_foreach
    (list=<optimized out>, func=func@entry=0x5560e2124180 <net_init_netdev>, opaque=opaque@entry=0x0, errp=errp@entry=0x7fff7e724bd0)
    at util/qemu-option.c:1171
#9  0x00005560e2126576 in net_init_clients (errp=0x7fff7e724bd0) at net/net.c:1564
#10 0x00005560e1ee7131 in main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at vl.c:4432
(gdb) q


Version-Release number of selected component (if applicable):
qemu-kvm-4.0.0-0.module+el8.1.0+3169+3c501422.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Boot a guest with a non-exist fd to tap device
-netdev tap,id=idR4I0Ft,vhost=on,vhostfd=22,fd=16 \
                                 ^^^^^^^^^^^^^^^^
2.
3.

Actual results:
QEMU core dump.

Expected results:
QEMU should give a kindly warning instead of core dump.

Additional info:
1. QEMU CML:
MALLOC_PERTURB_=1 \
/usr/libexec/qemu-kvm \
-S \
-name 'avocado-vt-vm1' \
-machine q35 \
-nodefaults \
-device VGA,bus=pcie.0,addr=0x1 \
-device pcie-root-port,id=pcie.0-root-port-4,slot=4,chassis=4,addr=0x4,bus=pcie.0 \
-device virtio-net-pci,mac=9a:91:92:93:94:95,id=id3cE67z,vectors=4,netdev=idR4I0Ft,bus=pcie.0-root-port-4,addr=0x0 \
-netdev tap,id=idR4I0Ft,vhost=on,vhostfd=22,fd=16 \
-m 14336 \
-smp 12,maxcpus=12,cores=6,threads=1,sockets=2 \
-cpu 'SandyBridge',+kvm_pv_unhalt \
-monitor stdio