Tell if a device supports a given PCI capability.
Returns the address of the requested capability structure within the
device's PCI configuration space or 0 in case the device does not
support it. Possible values for cap:
PCI_CAP_ID_PM Power Management
PCI_CAP_ID_AGP Accelerated Graphics Port
PCI_CAP_ID_VPD Vital Product Data
PCI_CAP_ID_SLOTID Slot Identification
PCI_CAP_ID_MSI Message Signalled Interrupts
PCI_CAP_ID_CHSWP CompactPCI HotSwap
PCI_CAP_ID_PCIX PCI-X