I can reproduce it on libvirt-3.9.0-14.el7_5.2.x86_64, it seems that the alias name only accept 24 characters

For 24 characters, guest start successfully
# virsh dumpxml rhel
...
<console type='pty'>
      <source path='/dev/pts/4'/>
      <target type='virtio' port='1'/>
      <alias name='ua-123456789012345678901234'/>
    </console>
...
# virsh start rhel
Domain rhel started

For 25 characters, vm can not start
# virsh dumpxml rhel
...
<console type='pty'>
      <target type='virtio' port='1'/>
      <alias name='ua-1234567890123456789012345'/>
    </console>
...
# virsh start rhel
error: Failed to start domain rhel
error: internal error: failed to format device alias for PTY retrieval

1. not only for virtio pty console and serial pty console and serial devices, all virtio channel do have such issue, including virtio channel with type=pty/vc/dev/file/pipe/stdio/udp/tcp/unix/spicevmc

# virsh dumpxml rhel | grep ua -A2 -B2
    <channel type='unix'>
      <target type='virtio' name='org.qemu.guest_agent.0'/>
      <alias name='ua-c60aba6e-b6d8-448b-ab6e-8c7b5c29f353'/>
      <address type='virtio-serial' controller='0' bus='0' port='1'/>
    </channel>

# virsh start rhel
error: Failed to start domain rhel
error: internal error: failed to format device alias for PTY retrieval

And some libvirtd.log for reference:

2018-03-28 03:34:40.927+0000: 8528: error : qemuProcessLookupPTYs:1946 : internal error: failed to format device alias for PTY retrieval
...
2018-03-28 04:54:59.774+0000: 8527: error : qemuProcessRefreshChannelVirtioState:2037 : internal error: failed to format device alias for PTY retrieval

2. channel with target type is guestfwd will ignore any alias setting.
# virsh edit rhel ===> to add alias like below
 <channel type='pty'>
      <target type='guestfwd' address='10.0.2.1' port='4600'/>
     <alias name='ua-justfortest'/>
    </channel>

# virsh dumpxml rhel
...
<channel type='pty'>
      <target type='guestfwd' address='10.0.2.1' port='4600'/>
    </channel>

==> no alias, the alias setting is ignored for channel with guestfwd target type


3. If we will add the alias name length check? I have tried a little, have not found the max length

1) set  <alias name='ua-'/> for interface device, vm can start with qemu option:

 -netdev tap,fd=27,id=hostua-,vhost=on,vhostfd=29 -device virtio-net-pci,netdev=hostua-,id=ua-,mac=52:54:00:03:20:4c,bus=pci.0,addr=0x3


2) tried with alias name as 'ua-{9027 characters}' with interface device, vm can start successfully

Patch proposed upstream:

https://www.redhat.com/archives/libvir-list/2018-March/msg01806.html

I've just pushed the patch upstream:

commit c4c32cb300beba41ecbca3ee4884c65630bde861
Author:     Michal Privoznik <mprivozn>
AuthorDate: Thu Mar 29 08:50:01 2018 +0200
Commit:     Michal Privoznik <mprivozn>
CommitDate: Sun Apr 1 12:38:38 2018 +0200

    qemu: Use dynamic buffer for storing PTY aliases
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1560976
    
    For historical reasons we've used 32 bytes long static buffer for
    storing PTY aliases. This breaks users scenario where they try to
    start a machine with user alias consisting of "ua-$uuid".
    
    Signed-off-by: Michal Privoznik <mprivozn>

v4.2.0-2-gc4c32cb300

test on libvirt-4.3.0-1.el7.x86_64, the result is as expected.

1. Set alias name for serial device in vm xml:
# virsh dumpxml rhel | grep /console -B9
    <serial type='pty'>
      <target type='isa-serial' port='0'>
        <model name='isa-serial'/>
      </target>
      <alias name='ua-3ea66e7d-40f6-4396-a3c0-3160c9bfdabe'/>
    </serial>
    <console type='pty'>
      <target type='serial' port='0'/>
      <alias name='ua-3ea66e7d-40f6-4396-a3c0-3160c9bfdabe'/>
    </console>
......
 <channel type='unix'>
      <source mode='bind' path='/var/lib/libvirt/qemu/channel/target/domain-8-rhel/org.qemu.guest_agent.0'/>
      <target type='virtio' name='org.qemu.guest_agent.0' state='disconnected'/>
      <alias name='ua-62e41900-52d9-4fe7-b788-c04dbb8e61e9'/>
      <address type='virtio-serial' controller='0' bus='0' port='1'/>
    </channel>

2. Start the vm
# virsh start rhel
Domain rhel started

3. Hotplug a virtio console, pci serial, virtio channel, then hot-unplug, all succeed
# cat console.xml
<console type='dev'>        
 <source path='/dev/ttyS0'/>      
 <target type='virtio' port='1'/> 
<alias name='ua-99d9ca12-dbd1-4c3f-b9c4-cfcccdad86aa'/>
</console>
# virsh attach-device rhel console.xml
Device attached successfully

# cat serial.xml
<serial type='pty'>
 <source path='/dev/pts/4'/>
 <target type='pci-serial' port='0'/> 
<alias name='ua-89a37c8e-363a-4936-b84d-863191eed07e'/>
</serial>
# virsh attach-device rhel serial.xml
Device attached successfully

# cat channel.xml
<channel type='pty'>       
<source path='/dev/pts/5'/>      
 <target type='virtio' name='arbitrary.virtio.serial.port.name'/>   
    <alias name='ua-9b8ebb59-093d-4f9c-b467-99a433e278a7'/>       
 </channel>
# virsh attach-device rhel channel.xml
Device attached successfully

# virsh dumpxml rhel 
==> check all devices exists with the alias name set

4. test duplicate alias
# virsh attach-device rhel ch.xml
error: Failed to attach device from ch.xml
error: XML error: non unique alias detected: ua-3ea66e7d-40f6-4396-a3c0-3160c9bfdabe

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHSA-2018:3113