libqos: Use explicit QTestState for fw_cfg operations

Drop one more client of global_qtest by teaching all fw_cfg test
functionality (invoked through alloc-pc) to pass in an explicit
QTestState, adjusting all callers.  In particular, fw_cfg-test
had to reorder things to create the test state prior to creating
the fw_cfg (and drop a pointless strdup in the meantime), but that
test now no longer depends on global_qtest.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
[thuth: Fixed conflict wrt pc_alloc_init() in vhost-user-test.c]
Signed-off-by: Thomas Huth <thuth@redhat.com>