qemu-iotests: Merge FilePaths and FilePath

qemu-iotests: Merge FilePaths and FilePath

FilePath creates now one temporary file:

    with FilePath("a") as a:

Or more:

    with FilePath("a", "b", "c") as (a, b, c):

This is also the behavior of the file_path() helper, used by some of the
tests. Now we have only 2 helpers for creating temporary files instead
of 3.

Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200828232152.205833-5-nsoffer@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>