OvmfPkg: Add support for qemu's -kernel parameter

OvmfPkg: Add support for qemu's -kernel parameter

If QEMU's -kernel parameter was used, then download the
kernel from the FwCfg interface, and launch it.  (See -kernel,
-initrd, -append) The application uses the LoadLinuxLib to boot
the kernel image.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@13923 6f19259b-4bc3-4df7-8a09-765794883524