OvmfPkg/PlatformInitLib: Add PlatformScanE820 and GetFirstNonAddressCB
First step replacing the PlatformScanOrAdd64BitE820Ram() function.
Add a PlatformScanE820() function which loops over the e280 entries
from FwCfg and calls a callback for each of them.
Add a GetFirstNonAddressCB() function which will store the first free
address (right after the last RAM block) in
PlatformInfoHob->FirstNonAddress. This replaces calls to
PlatformScanOrAdd64BitE820Ram() with non-NULL MaxAddress.
Write any actions done (setting FirstNonAddress) to the firmware log
with INFO loglevel.
Also drop local FirstNonAddress variables and use
PlatformInfoHob->FirstNonAddress instead everywhere.