OvmfPkg: PlatformBootManagerLibGrub: Allow executing kernel via fw_cfg

OvmfPkg: PlatformBootManagerLibGrub: Allow executing kernel via fw_cfg

Support QEMU's -kernel option.

Create a QemuKernel.c for PlatformBootManagerLibGrub
which is an exact copy of the file
PlatformBootManagerLib/QemuKernel.c .

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ashish Kalra <ashish.kalra@amd.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Erdem Aktas <erdemaktas@google.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Min Xu <min.m.xu@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3457
Signed-off-by: James Bottomley <jejb@linux.ibm.com>
Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>