Fix two issues:

Fix two issues:
1. The default with of hex value is 0 according to MdeLib spec. If we expect it to be 16, we need to state it in the format string explicitly.
2. For UnicodeSPrint, the output buffer size of 0 will not print anything to the buffer according to MdeLib spec. We need to specify the maximum possible buffer size.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@4635 6f19259b-4bc3-4df7-8a09-765794883524