Enable reinitialization of the libpciaccess global state, necessary
to correctly handle dynamic add/remove of PCI devices, such as the
creation/removal of Virtual Functions (VFs). Update
igt_pci_system_cleanup() to conditionally call pci_system_cleanup()
based on the initialization state. Introduce igt_pci_system_reinit()
for explicit reinitialization of the libpciaccess global state,
particularly useful after PCI device changes, to be used in
subsequent patches.

v2:
- Change pci_system_initialized type from int to bool for consistency
  with install_handler (Peter)
- Simplify conditional logic in igt_pci_system_init by removing
  redundant assignments

Signed-off-by: Marcin Bernatowicz <marcin.bernatowicz@linux.intel.com>
Reviewed-by: Peter Senna Tschudin <peter.senna@linux.intel.com>
Cc: Adam Miszczak <adam.miszczak@linux.intel.com>
Cc: Lukasz Laguna <lukasz.laguna@intel.com>
Cc: Jakub Kolakowski <jakub1.kolakowski@intel.com>
Cc: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Cc: Chris Wilson <chris.p.wilson@intel.com>
Cc: Peter Senna Tschudin <peter.senna@linux.intel.com>