OvmfPkg/VirtioPciDeviceDxe: supply missing BUS_MASTER attribute
    
    Virtio devices read and write guest RAM (they don't just decode their IO
    and/or MMIO BARs), which translates to "bus master".
    
    Suggested-by: Laszlo Ersek <lersek@redhat.com>
    Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
    Cc: Jordan Justen <jordan.l.justen@intel.com>
    Cc: Tom Lendacky <thomas.lendacky@amd.com>
    Cc: Laszlo Ersek <lersek@redhat.com>
    Contributed-under: TianoCore Contribution Agreement 1.1
    Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
    [lersek@redhat.com: expand commit message body]
    [lersek@redhat.com: fix up line breaking style (original code too)]
    Reviewed-by: Laszlo Ersek <lersek@redhat.com>
    Regression-tested-by: Laszlo Ersek <lersek@redhat.com>