OvmfPkg/VirtioMmioDeviceLib: improve style of mMmioDeviceProtocolTemplate

In edk2, we spell "static" "STATIC", plus objects with static storage
duration (esp. protocol templates) should be const-qualified (spelled
"CONST") whenever possible.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>