tests/qtest/ahci-test.c: Calculate iso_size with 64-bit arithmetic

tests/qtest/ahci-test.c: Calculate iso_size with 64-bit arithmetic

Coverity notes that when calculating the 64-bit iso_size value in
ahci_test_cdrom() we actually only do it with 32-bit arithmetic.
This doesn't matter for the current test code because nsectors is
always small; but adding the cast avoids the coverity complaints.

Fixes: Coverity CID 1432343
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-Id: <20210506194358.3925-1-peter.maydell@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>