OvmfPkg: VirtioRngDxe: clear all feature bits more explicitly

OvmfPkg: VirtioRngDxe: clear all feature bits more explicitly

This too is in preparation for the following patches.

After this patch, all four drivers manage their feature bits with explicit
masking.

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>