tests: fix iptables test case commandline options in virfirewalltest.c

tests: fix iptables test case commandline options in virfirewalltest.c

This test was created with all the commandlines erroneously having
"--source-host", which is not a valid iptables option. The correct
name for the option is "--source". However, since the test is just
checking that the generated commandline matches what we told it to
generate (and never actually runs iptables, as that would be a "Really
Bad Idea"(tm)), the test has always succeeded. I only found it because
I made a change to the code that caused the test to incorrectly try to
run iptables during the test, and the error message I received was
"odd" (it complained about the bad option, rather than complaining
that I had insufficient privilege to run the command).

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>