OvmfPkg/VirtioGpuDxe: map virtio GPU command objects to device addresses
    
    Every virtio GPU command used by VirtioGpuDxe is synchronous and formatted
    as a two-descriptor chain: request, response. The internal workhorse
    function that all the command-specific functions call for such messaging
    is VirtioGpuSendCommand().
    
    In VirtioGpuSendCommand(), map the request from system memory to bus
    master device address for BusMasterRead operation, and map the response
    from system memory to bus master device address for BusMasterWrite
    operation.
    
    Pass the bus master device addresses to VirtioAppendDesc(). (See also
    commit 4b725858de68, "OvmfPkg/VirtioLib: change the parameter of
    VirtioAppendDesc() to UINT64", 2017-08-23.)
    
    Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
    Cc: Brijesh Singh <brijesh.singh@amd.com>
    Cc: Jordan Justen <jordan.l.justen@intel.com>
    Cc: Tom Lendacky <thomas.lendacky@amd.com>
    Contributed-under: TianoCore Contribution Agreement 1.1
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>