Error (12700)
VMM cannot complete the Hyper-V operation on the [virtualhost.domain.com] server because of the error: ‘[VIRTUALGUEST]‘ failed to add device ‘Microsoft Emulated Ethernet Port’. (Virtual machine ID 26DD8B56-0594-446A-8084-405218737EB0)

The Virtual Machines configuration 26DD8B56-0594-446A-8084-405218737EB0 at ‘J:\VS\[VIRTUALGUEST]‘ is no longer accessible: The requested operation cannot be performed on a file with a user-mapped section open. (0x800704C8)
(Unknown error (0×8000))

Recommended Action
Resolve the issue in Hyper-V and then try the operation again.

Well, that’s really good advice – clear the error and try again.  So, a quick Google-ing turned up a Microsoft KB article: Creating or starting a Hyper-V virtual machine on Windows Server 2008 or Microsoft Hyper-V Server 2008 may fail with error: 0x800704C8.  Oddly enough, this was exactly my issue:

CAUSE

This issue can be caused by antivirus software that is installed in the parent partition and the real-time scanning component is configured to monitor the Hyper-V virtual machine files.

RESOLUTION

To resolve this issue, configure the real-time scanning component within the antivirus software to exclude the following directories and files:

• Default virtual machine configuration directory (C:\ProgramData\Microsoft\Windows\Hyper-V)
• Custom virtual machine configuration directories
• Default virtual hard disk directory (C:\Users\Public\Documents\Hyper-V\Virtual Hard Disks)
• Custom virtual hard disk directories
• Snapshot directories
• Vmms.exe
• Vmwp.exe

Notes:

• If virtual machines are missing from the Hyper-V Management console, you must configure the antivirus exclusions, and then restart the Hyper-V Virtual Machine Management service.
• If the error code was 0x800704C8, it is likely that the virtual machine configuration file was corrupted and the virtual machine may need to be re-created or restored from backup if restarting the Hyper-V Virtual Machine Management service does not resolve the issue.

Chalk one up for Microsoft KB articles.  Now, a little further down in the KB article, it suggests (per Planning for Hyper-V Security) installing the antivirus on the guest (which we do) and not on the host (which we do, too):

Do not run any applications in the parent partition. Run all applications on virtual machines, which use child partitions. For example, if antivirus is required, be sure to run it on the VMs rather than the parent partition. Keeping the parent partition free of applications and running on a Windows Server 2008 core installation means fewer host updates, since nothing needs software updates except the Windows Server 2008 core installation, the Hyper-V service components, and the small (~600KB) hypervisor.