OvmfPkg: VirtioBlkDxe: don't clear non-negotiable feature bits

OvmfPkg: VirtioBlkDxe: don't clear non-negotiable feature bits

VirtioBlkDxe only recognizes virtio-block feature bits that the device
offers non-negotiably. Nonetheless, in preparation for the following
patches, don't try to clear them even for simplicity.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>