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.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>