tests/functional: Fix hash validation

tests/functional: Fix hash validation

The _check() function is supposed to check whether the hash of the
downloaded file matches the expected one. Unfortunately, during the
last rework of this function, the check was accidentally turned into
returning the hash value itself instead of a True/False value,
effectively accepting each hash as valid. Let's do a proper check
again now.

Fixes:05e303210d ("tests/functional/qemu_test: Use Python hashlib ...")
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>