vfio/pci: Pass an error object to vfio_add_capabilities

vfio/pci: Pass an error object to vfio_add_capabilities

Pass an error object to prepare for migration to VFIO-PCI realize.
The error is cascaded downto vfio_add_std_cap and then vfio_msi(x)_setup,
vfio_setup_pcie_cap.

vfio_add_ext_cap does not return anything else than 0 so let's transform
it into a void function.

Also use pci_add_capability2 which takes an error object.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>