Improve deliberate test coverage for addressmap_get_virtual_address()

This function is currently tested indirectly from test_entryconn.c, but this results in unreliable coverage for a couple of the features in this function.

Specifically, we don't test whether duplicate addresses are rejected, or whether IPv4 addresses ending with .0 or .255 are rejected.

This is one source of nondeterminism in our test coverage.