OvmfPkg/NvVarsFileLib: Shortcut ConnectNvVarsToFileSystem in secure-boot

OvmfPkg/NvVarsFileLib: Shortcut ConnectNvVarsToFileSystem in secure-boot

OvmfPkg/Library/NvVarsFileLib allows loading variables into emulated
varstore from a on-disk NvVars file.  We can't allow that when secure
boot is active.  So check secure-boot feature and shortcut the
ConnectNvVarsToFileSystem() function when sb is enabled.

Cc: Erdem Aktas <erdemaktas@google.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Suggested-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Min Xu <min.m.xu@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>