Patches proposed upstream:

https://www.redhat.com/archives/libvir-list/2018-December/msg00069.html

I've just pushed the patch upstream:

commit 7730a2ead4279e3a11771e1069096747988410cf
Author:     Michal Privoznik <mprivozn>
AuthorDate: Tue Dec 4 14:30:37 2018 +0100
Commit:     Michal Privoznik <mprivozn>
CommitDate: Wed Dec 5 15:07:29 2018 +0100

    qemuDomainRemoveRNGDevice: Remove associated chardev too
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1656014
    
    An RNG device can consists of more devices than RND device
    itself. For instance, in case of EGD there is a chardev that
    connects to EGD daemon and feeds the qemu with random data. When
    doing RNG device removal we have to remove the associated chardev
    as well.
    
    Signed-off-by: Michal Privoznik <mprivozn>
    Reviewed-by: Ján Tomko <jtomko>

v4.10.0-24-g7730a2ead4

Reproduced on latest rhel7.6.z:
libvirt-4.5.0-10.el7_6.3.x86_64
qemu-kvm-rhev-2.12.0-19.el7_6.2.x86_64

Follow steps in comment 0, get error:
# virsh attach-device rhel7.6 udp-connect.xml
error: Failed to attach device from udp-connect.xml
error: internal error: unable to execute QEMU command 'chardev-add': attempt to add duplicate property 'charrng0' to object (type 'container')

Verify this bug with:
libvirt-daemon-5.0.0-11.module+el8.0.1+3459+e357ef2f.x86_64
qemu-kvm-3.1.0-27.module+el8.0.1+3253+c5371cb3.x86_64

(Egd rng devices are hard to be removed due to Bug 1543416, and if with udp rng dev, rhel8 guest os can not be accessed due to Bug 1660802, except "tcp connect mode rng dev". So using this one as an example to verify this bug.)

Steps:
1. Start a guest with tcp connect mode egd rng device:
# cat /dev/urandom |nc -l 1024

# virsh start avocado-vt-vm1
Domain avocado-vt-vm1 started

Check qemu cli, 3 parameters for egd rng dev: "chardev, object, device"
# ps aux|grep qemu|grep rng
... -chardev socket,id=charrng0,host=localhost,port=1024 -object rng-egd,id=objrng0,chardev=charrng0 -device virtio-rng-pci,rng=objrng0,id=rng0,bus=pci.6,addr=0x0

# virsh dumpxml avocado-vt-vm1|grep rng -3
...
    <rng model='virtio'>
      <backend model='egd' type='tcp'>
        <source mode='connect' host='localhost' service='1024' tls='no'/>
        <protocol type='raw'/>
      </backend>
      <alias name='rng0'/>
      <address type='pci' domain='0x0000' bus='0x06' slot='0x00' function='0x0'/>
    </rng>
...
[In guest]# python3
>>> import os
>>> f=os.open('/dev/hwrng', os.O_RDONLY)
>>> os.read(f,10)
b'iI\x0c\xc0bN\x8e\x80\x11\x7f'
>>> 

2. Hot-unplug the egd rng device:
# cat rng-tcp.xml 
    <rng model='virtio'>
      <backend model='egd' type='tcp'>
        <source mode='connect' host='localhost' service='1024'/>
        <protocol type='raw'/>
      </backend>
    </rng>

# virsh detach-device avocado-vt-vm1 rng-tcp.xml 
Device detached successfully

# virsh dumpxml avocado-vt-vm1|grep rng -3

[In guest]# cat /dev/hwrng
cat: /dev/hwrng: No such device

Check libvirtd.log, 3 are all deleted separately by: "qemuMonitorDelDevice, qemuMonitorDelObject, qemuMonitorDetachCharDev"
# cat libvirt.log-rng |grep -E "Del|CharDev"
2019-07-08 09:04:00.516+0000: 9286: debug : ***qemuMonitorDelDevice***:3011 : devalias=rng0
2019-07-08 09:04:00.516+0000: 9286: debug : qemuMonitorDelDevice:3013 : mon:0x7f096c0d23c0 vm:0x7f09141ae4d0 json:1 fd:30
2019-07-08 09:04:06.912+0000: 9282: debug : qemuMonitorJSONHandleDeviceDeleted:1127 : missing device in device deleted event
2019-07-08 09:04:06.965+0000: 9282: debug : qemuMonitorEmitDeviceDeleted:1544 : mon=0x7f096c0d23c0
2019-07-08 09:04:06.965+0000: 9282: debug : qemuProcessHandleDeviceDeleted:1301 : Device rng0 removed from domain 0x7f09141ae4d0 avocado-vt-vm1
2019-07-08 09:04:06.966+0000: 9920: debug : processDeviceDeletedEvent:4278 : Removing device rng0 from domain 0x7f09141ae4d0 avocado-vt-vm1
2019-07-08 09:04:06.966+0000: 9920: debug : ***qemuMonitorDelObject***:3187 : objalias=objrng0
2019-07-08 09:04:06.966+0000: 9920: debug : qemuMonitorDelObject:3189 : mon:0x7f096c0d23c0 vm:0x7f09141ae4d0 json:1 fd:30
2019-07-08 09:04:06.971+0000: 9920: debug : ***qemuMonitorDetachCharDev***:4004 : chrID=charrng0
2019-07-08 09:04:06.972+0000: 9920: debug : qemuMonitorDetachCharDev:4006 : mon:0x7f096c0d23c0 vm:0x7f09141ae4d0 json:1 fd:30


3.Hotplug the egd rng device again:
# cat /dev/urandom |nc -l 1024

# virsh attach-device avocado-vt-vm1 rng-tcp.xml 
Device attached successfully



Additional info:
1.With same steps, bug can be reproduced on rhel7.6 with error below in step3:
 "unable to execute QEMU command 'chardev-add': attempt to add duplicate property 'charrng0' to object (type 'container')"
And in step2, will see from libvirtd.log, no "qemuMonitorDetachCharDev" action when detach the egd rng device.
#  cat libvirtd.log-rng |grep -E "Del|CharDev"
2019-07-08 11:20:38.429+0000: 28197: debug : ***qemuMonitorDelDevice***:2976 : devalias=rng0
2019-07-08 11:20:38.429+0000: 28197: debug : qemuMonitorDelDevice:2978 : mon:0x7fab240137f0 vm:0x7faad82a7940 json:1 fd:29
2019-07-08 11:20:38.670+0000: 28192: debug : qemuMonitorJSONHandleDeviceDeleted:1102 : missing device in device deleted event
2019-07-08 11:20:38.719+0000: 28192: debug : qemuMonitorEmitDeviceDeleted:1544 : mon=0x7fab240137f0
2019-07-08 11:20:38.719+0000: 28192: debug : qemuProcessHandleDeviceDeleted:1279 : Device rng0 removed from domain 0x7faad82a7940 avocado-vt-vm1
2019-07-08 11:20:38.719+0000: 28197: debug : ***qemuMonitorDelObject***:3152 : objalias=objrng0
2019-07-08 11:20:38.719+0000: 28197: debug : qemuMonitorDelObject:3154 : mon:0x7fab240137f0 vm:0x7faad82a7940 json:1 fd:29

This was verified and shipped long ago. Closing the bug report.