Fix setupDiskImages when the devices are already in the tree.
Run fsck before obtaining minimum filesystem size. (#1162215)

resize2fs -P now requires an e2fsck -f first. The lack of a real minimum
size left us with no useful minInstanceSize and allowed users to attempt
a shrink down to any size, which led to failures.

If we have a currentSize, set the default minInstanceSize to that so
that a failure to obtain a true minInstanceSize from the fs tools
precludes shrinking the filesystem.
Disable resize operations on filesystems whose current size is unknown.

It is not safe to attempt to resize without knowing the current size
or to attempt to shrink without knowing the true minimum size.

Related: rhbz#1162215
Fix units for fs min size padding.
New method to round a Size to a whole number of a specified unit.
Round filesystem target size to whole resize tool units. (#1163410)

Add a check in case the rounding changes the direction of resize.