OvmfPkg: PciHostBridgeDxe: factor out InitRootBridge() function

OvmfPkg: PciHostBridgeDxe: factor out InitRootBridge() function

This new function incorporates the current loop body found in the entry
point function, InitializePciHostBridge(). It will be called once for each
root bus discovered.

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@17958 6f19259b-4bc3-4df7-8a09-765794883524