OvmfPkg/VirtioLib: change the parameter of VirtioAppendDesc() to UINT64

OvmfPkg/VirtioLib: change the parameter of VirtioAppendDesc() to UINT64

The patch change the "BufferPhysAddr" parameter of VirtioAppendDesc()
from type UINTN to UINT64.

UINTN is appropriate as long as we pass system memory references. After
the introduction of bus master device addresses, that's no longer the case
in general. Should we implement "real" IOMMU support at some point, UINTN
could break in 32-bit builds of OVMF.

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: clarify commit message]
[lersek@redhat.com: balance parens in VirtioAppendDesc() comment blocks]
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>