qemu: pci_hotplug - Introduce hotplug_timeout

If we set the global test timeout as the timeout for
the hotplug operation, if it is reached due to a QEMU
bug the test will never end properly while executing
under autotest. So enforce a specific timeout,
a lot smaller than the typical global test timeout.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>