iotest 055: refactor and speed up

Source disk is created and filled with test data before each test case.
Instead initialize it once for the whole unit.

Test disk filling patterns are merged into one pattern.

Also TestSetSpeed used different image_len for source and target (by
mistake) - this is automatically fixed here.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 1470748523-13856-1-git-send-email-vsementsov@virtuozzo.com
Reviewed-by: Pavel Butsykin <pbutsykin@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
