Do not upgrade Hyper-V instances from 10.x to 11.x yet

[Alexander Shitov]

11.0-RELEASE has issues with Hyper-V compatibility. It does not
recognizes and then detaches virtual disk.

storvsc0: <Hyper-V IDE Storage Interface> on vmbus0
(probe0:blkvsc0:0:0:0): storvsc scsi_status = 2
(probe0:blkvsc0:0:1:1): invalid LUN 1
...
(probe0:blkvsc0:0:0:1): invalid LUN 1
da0 at blkvsc0 bus 0 scbus1 target 0 lun 0
da0: <specs>
da0 at blkvsc0 bus 0 scbus1 target 0 lun 0
da0: <Msft Virtual Disk 1.0> detached

Unfortunately bug affects possibility to upgrade existing instances
and prevents new 11.0 installations in Hyper-V environment.
PR: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=212721
I guess that this might be fixed with an errata notice after the
release announcement.

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Best Regards,
Alexander