When encountering an LVM2 layout, activate all its logical volumes
so that they can be mounted.
NB: we need to pass "norecovery" to the mount options because LVM
does not setup the device mapper tables read-only even though the
underlying loopback device is and then xfs will try to write to
its journal and the kernel will panic. Attempts to reload the DM
tables as readonly didn't work.
NB: this will not work if we are trying to inspect an image that
has a volume group name that is also present on the host. We
could open the image file read-write and modify its vg name, but
that would mean modifying the image file and thus we would need
to copy it first.
Pass `-c /dev/null` to `blkid` to force it not to use its cache.
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