This and a few other corner case checks were added by commits:

commit a83b2e253f0750c7053e2bf0f4510d6e1de73264
Author: Peter Krempa <pkrempa>
Date:   Tue Apr 28 17:33:54 2015 +0200

    qemu: Validate available slot count for memory devices
    
    While qemu would reject the configuration we can check whether it makes
    sense to plug the device upfront.

commit 6705d828fc44fabf9f24c73a649089f068b00f35
Author: Peter Krempa <pkrempa>
Date:   Tue Apr 28 17:15:43 2015 +0200

    qemu: command: Validate that memory devices slot ID is in range
    
    slot id, if specified, has to be less than the slots count.

commit 406944e476729ce740da126d767b073e23c1af4d
Author: Peter Krempa <pkrempa>
Date:   Tue Apr 28 16:23:05 2015 +0200

    qemu: conf: Reject memory device if it would exceed configured max size
    
    If the added memory device would exceed the maximum memory size, reject
    it.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1216046

v1.2.15-rc1-22-ga83b2e2

Hi Peter,

I have found another guest disappear because memory vs current memory (and test with upstream libvirt, this issue still exist), would you please help to check out if need open a another bug to track it? thanks in advance for your help.

1. prepare a vm with memory deivce like this:

# virsh dumpxml test3

  <maxMemory slots='16' unit='KiB'>2560000</maxMemory>
  <memory unit='KiB'>1536000</memory>
  <currentMemory unit='KiB'>1536000</currentMemory>
...
  <cpu>
    <numa>
      <cell id='0' cpus='0-1' memory='512000' unit='KiB'/>
      <cell id='1' cpus='2-3' memory='512000' unit='KiB'/>
    </numa>
  </cpu>
...
    <memory model='dimm'>
      <target>
        <size unit='KiB'>512000</size>
        <node>0</node>
      </target>
    </memory>

2. cold-unplug the memory deivce:
# virsh detach-device test3 memdevice.xml --config
Device detached successfully

# virsh dumpxml test3
  <maxMemory slots='16' unit='KiB'>2560000</maxMemory>
  <memory unit='KiB'>1024000</memory>
  <currentMemory unit='KiB'>1536000</currentMemory>


3. restart libvirtd and vm will disappear

# service libvirtd restart
Redirecting to /bin/systemctl restart  libvirtd.service

# virsh edit test3
error: failed to get domain 'test3'
error: Domain not found: no domain with matching name 'test3'


And ask another question about the display of initial memory and memory.

I test memory device with qemu-2.3.0, I found the current memory is not correct if we hot-plug a memory device:

1. prepare a running vm with maxmemory and numa settings: 

  <maxMemory slots='16' unit='KiB'>2560000</maxMemory>
  <memory unit='KiB'>1024000</memory>
  <currentMemory unit='KiB'>1024000</currentMemory>
...
  <cpu>
    <numa>
      <cell id='0' cpus='0-1' memory='512000' unit='KiB'/>
      <cell id='1' cpus='2-3' memory='512000' unit='KiB'/>
    </numa>
  </cpu>

2. hot-plug a memory device
# virsh attach-device test3 memdevice.xml
Device attached successfully

3. check the current memory and memory ballon:

# virsh dumpxml test3
...
  <maxMemory slots='16' unit='KiB'>2560000</maxMemory>
  <memory unit='KiB'>1536000</memory>
  <currentMemory unit='KiB'>1024000</currentMemory>


# virsh dommemstat test3
actual 1536000
swap_in 0
swap_out 0
major_fault 370
minor_fault 393570
unused 1375984
available 1506908
rss 260156


# virsh qemu-monitor-command test3 --hmp info balloon
balloon: actual=1500

and check in guest the actual memory is 1471M. And from libvirt.org current memory is The actual allocation of memory for the guest. So this value(current memory) should not be 1536000 after hot-plug memory deivce?

After talking with peter via IRC, open a new bug 1220702 to track the issue in comment 2.

Verify this bug with libvirt-1.2.17-2.el7.x86_64:

1.

# virsh dumpxml rhel7.0 |grep -A3 maxMemory
  <maxMemory slots='16' unit='KiB'>2560000</maxMemory>
  <memory unit='KiB'>1024000</memory>
  <currentMemory unit='KiB'>1024000</currentMemory>
  <vcpu placement='static'>4</vcpu>

2.

# virsh attach-device rhel7.0 memdevice.xml --config
Device attached successfully

# virsh attach-device rhel7.0 memdevice.xml --config
Device attached successfully

# virsh attach-device rhel7.0 memdevice.xml --config
error: Failed to attach device from memdevice.xml
error: unsupported configuration: Attaching memory device with size '524288' would exceed domain's maxMemory config

3. recheck the xml :

# virsh dumpxml rhel7.0 |grep -A3 maxMemory
  <maxMemory slots='16' unit='KiB'>2560000</maxMemory>
  <memory unit='KiB'>2072576</memory>
  <currentMemory unit='KiB'>2072576</currentMemory>



4. edit xml change the slot:

# virsh dumpxml rhel7.0 |grep -A3 maxMemory
  <maxMemory slots='1' unit='KiB'>2560000</maxMemory>
  <memory unit='KiB'>1024000</memory>
  <currentMemory unit='KiB'>1024000</currentMemory>
  <vcpu placement='static'>4</vcpu>

# virsh attach-device rhel7.0 memdevice.xml --config
Device attached successfully

# virsh attach-device rhel7.0 memdevice.xml --config
error: Failed to attach device from memdevice.xml
error: unsupported configuration: no free memory device slot available

5. start a guest with a memory device which slot is out of range:

# virsh dumpxml rhel7.0
  <maxMemory slots='1' unit='KiB'>2560000</maxMemory>
  <memory unit='KiB'>1286144</memory>
  <currentMemory unit='KiB'>1286144</currentMemory>
...
    <memory model='dimm'>
      <target>
        <size unit='KiB'>262144</size>
        <node>0</node>
      </target>
      <address type='dimm' slot='2' base='0x0'/>
    </memory>

# virsh start rhel7.0
error: Failed to start domain rhel7.0
error: unsupported configuration: memory device slot '2' exceeds slots count '1'

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://rhn.redhat.com/errata/RHBA-2015-2202.html