OvmfPkg/PvScsiDxe: Fix VS2019 build error because of implicit cast

OvmfPkg/PvScsiDxe: Fix VS2019 build error because of implicit cast

Sean reported that VS2019 build produce the following build error:
INFO - PvScsi.c
INFO - Generating code
INFO - d:\a\1\s\OvmfPkg\PvScsiDxe\PvScsi.c(459): error C2220: the
       following warning is treated as an error
INFO - d:\a\1\s\OvmfPkg\PvScsiDxe\PvScsi.c(459): warning C4244: '=':
       conversion from 'const UINT16' to 'UINT8', possible loss of data

This result from an implicit cast from PVSCSI Response->ScsiStatus
(Which is UINT16) to Packet->TargetResponse (Which is UINT8).

Fix this issue by adding an appropriate explicit cast and verify with
assert that this truncation do not result in loss of data.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2651
Reported-by: Sean Brogan <sean.brogan@microsoft.com>
Signed-off-by: Liran Alon <liran.alon@oracle.com>
Message-Id: <20200331110452.51992-1-liran.alon@oracle.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
[lersek@redhat.com: rewrap VS2019 diags in commit msg for PatchCheck.py]