Fixes Hyper-V volume mapping issue on reboot

Hyper-V does not manage iSCSI mounted passthrough disks
correctly on reboot in some circumstances where disk
number assignment can change, resulting in instances potentially
booting with the wrong volume attached.

Co-Authored-By: Adelina Tuvenie <atuvenie@cloudbasesolutions.com>
Co-Authored-By: Adrian Bora <abora@cloudbasesolutions.com>

Change-Id: I99fa75aaa950de2ada8a03bd23c910c3ff810d69
Closes-Bug: #1322926
disk/vfs: ensure guestfs capabilities

Ensures that guestfs is available and well configured. Also
when checking for capabilities this commit introduces an in-memory
flag to avoid repeating the process if succeeded.
If we are able to load guestfs but something wrong happens a fatal
error is raised.

DocImpact: We should add a not for ubuntu like systems that use
libguestfs to execute the command 'update-guestfs-appliance' for
configuring it.

Closes-Bug: #1275267
Closes-Bug: #1157922
Change-Id: I97b3a23829ea1f3aadfe08ca6448b35010d2f312
Fix DB migration 254 by adding missing unittest

By merging https://review.openstack.org/#/c/114286/23, a new db migration
was added but the unittest was missing.

Change-Id: I32e2455fd5efb2aad52d9a4d7666783c0e613e75
Merge "Fixes Hyper-V volume mapping issue on reboot"
Merge "disk/vfs: ensure guestfs capabilities"
Merge "Fix DB migration 254 by adding missing unittest"