Manage backing store more independently in loop backed test cases.
Manage backing store more independently in loop backed test cases.

* Make the backing store before starting to use the loop devices so there
is less elapsed time between finding a free loop device and actually using
it. Get rid of it if there is a failure in locating a free loop device
or using it.

* Make self._loopMap a list of pairs, since we never do random access
anyway.

* Don't give up on deleting all loop devices the first time one loop
device is not succesfully removed.

* Give every loop device a minimum of three deletion attempts
before teardown gives up. This is apallingly ad-hoc, but almost works.
As many deletion attempts as needed is sometimes infinitely too many.

* Use tempfile.NamedTemporaryFile to get backing store. Still put the
tempfile in "/var/tmp" not "/tmp".

Signed-off-by: mulhern <7b51bcf507bcd7afb72bf8663752c0ddbeb517f6@redhat.com>

rvykydal/blivet,jkonecny12/blivet,jkonecny12/blivet,AdamWill/blivet,rvykydal/blivet,vpodzime/blivet,vojtechtrefny/blivet,vojtechtrefny/blivet,rhinstaller/blivet,AdamWill/blivet,rhinstaller/blivet,vpodzime/blivet