OvmfPkg: EFI handover flags are in Bp->hdr.xloadflags

OvmfPkg: EFI handover flags are in Bp->hdr.xloadflags

LoadLinux() is looking at the wrong field for the kernel's EFI handover
protocol flags. It's not currently possible for JumpToUefiKernel() to
ever be called (even accidentally) because BIT2 and BIT3 of
Bp->hdr.load_flags are never set in modern kernels, which means that
control is always transferred to the kernel via the legacy entry point.

Look at the correct field so that the EFI handover protocol is used
whenever it's available.

Contributed-under: TianoCore Contribution Agreement 1.0
Cc: David Woodhouse <David.Woodhouse@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14721 6f19259b-4bc3-4df7-8a09-765794883524