OvmfPkg/QemuFlashFvbServicesRuntimeDxe: Restore C-bit when SEV is active

AmdSevDxe maps the flash memory range with C=0, but
SetMemorySpaceAttributes() unconditionally resets the C-bit to '1'. Lets
restore the mapping back to C=0.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Julien Grall <julien.grall@linaro.org>
Cc: Justen Jordan L <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>