OvmfPkg/QemuKernelLoaderFsDxe: call VerifyBlob after fetch from fw_cfg

OvmfPkg/QemuKernelLoaderFsDxe: call VerifyBlob after fetch from fw_cfg

In QemuKernelLoaderFsDxeEntrypoint we use FetchBlob to read the content
of the kernel/initrd/cmdline from the QEMU fw_cfg interface.  Insert a
call to VerifyBlob after fetching to allow BlobVerifierLib
implementations to add a verification step for these blobs.

This will allow confidential computing OVMF builds to add verification
mechanisms for these blobs that originate from an untrusted source
(QEMU).

The null implementation of BlobVerifierLib does nothing in VerifyBlob,
and therefore no functional change is expected.

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
Co-developed-by: James Bottomley <jejb@linux.ibm.com>
Signed-off-by: James Bottomley <jejb@linux.ibm.com>
Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>