OvmfPkg: PlatformBdsLib: connect all PCI root buses

OvmfPkg: PlatformBdsLib: connect all PCI root buses

Currently we only connect the root bus with bus number 0, by device path.
Soon we will possibly have several extra root buses, so connect all root
buses up-front (bus number zero and otherwise), by protocol GUID.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Regression-tested-by: Gabriel Somlo <somlo@cmu.edu>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17954 6f19259b-4bc3-4df7-8a09-765794883524