This reverts commit 8000e76d43, because
as @marmarek pointed out, the original was correct and I totally
misread. The check in question is checking whether to _abort_, not
whether to continue. So we want to check if the block device size is
_less_ than the filesystem + margin, not more.
ReopensQubesOS/qubes-issues#4553
If xvda is not partitioned, checking size of device mounted as root dev
doesn't detect xvda resize - it will have the new size, while the fs on
it not. Change to checking actual filesystem size.
Also, improve log message to include the size.
FixesQubesOS/qubes-issues#4274