OvmfPkg/VmgExitLib: Check for an explicit DR7 cached value
    
    BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3108
    
    Check the DR7 cached indicator against a specific value. This makes it
    harder for a hypervisor to just write random data into that field in an
    attempt to use an invalid DR7 value.
    
    Cc: Jordan Justen <jordan.l.justen@intel.com>
    Cc: Laszlo Ersek <lersek@redhat.com>
    Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
    Cc: Brijesh Singh <brijesh.singh@amd.com>
    Reviewed-by: Laszlo Ersek <lersek@redhat.com>
    Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
    Message-Id: <65157c1155a9c058c43678400dfc0b486e327a3e.1610045305.git.thomas.lendacky@amd.com>