ivshmem-test: Drop dependence on global_qtest

ivshmem-test: Drop dependence on global_qtest

Managing parallel connections to two different monitors via
the implicit global_qtest makes it hard to copy-and-paste code
to tests that are not aware of the implicit state.  Since we
have already fixed qpci to avoid global_qtest, we can now
simplify by not using global_qtest anywhere in ivshmem-test.

We can assert that the conversion is correct by checking that
global_qtest remains NULL throughout the test (a later patch
that changes global_qtest to not be a public global variable
will drop the assertions).

Signed-off-by: Eric Blake <eblake@redhat.com>
[thuth: Dropped the changes to test_ivshmem_hotplug() - will be fixed later]
Signed-off-by: Thomas Huth <thuth@redhat.com>