Fix bugs in the PCI bus driver to support SR-IOV.

Fix bugs in the PCI bus driver to support SR-IOV.
1. Expand the type of Offset in the _PCI_BAR structure from UINT8 to UINT16, because a VF BAR’s offset may be >= 0x100;
2. Enable ARI Capable Hierarchy for SR-IOV devices at earlier time because FirstVFOffset and VFStride of a SR-IOV device may change after its ARI Capable Hierarchy is set;
3. Change type of PcdSrIovSupport, PcdAriSupport, PcdMrIovSupport from FeatureFlag to [FixAtBuild, PcdDynamics], which allows SR-IOV/MR-IOV/ARI feature can be turn on/off dynamically, typically via a setup option.
4. Change PCI bus scan algorithm in PciScanBus() to prevent the case where some ARI extended functions may be skipped in the scan loop.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@10644 6f19259b-4bc3-4df7-8a09-765794883524