vmtests: add option to tar disk images after test run
vmtests may create a number of disk image files for use, these
are all sparse disks which report a size larger than the actual
space they occupy. In some cases someone copying files may
not be aware of the sparseness and expand the images instead.
Setting CURTIN_VMTEST_TAR_DISKS=1 will trigger vmtest to tar
sparsely the files and write out a .tar file in the same path.
This flag is disabled by default.