OvmfPkg/PvScsiDxe: Report the number of targets and LUNs

OvmfPkg/PvScsiDxe: Report the number of targets and LUNs

Implement EXT_SCSI_PASS_THRU.GetNextTarget() and
EXT_SCSI_PASS_THRU.GetNextTargetLun().

ScsiBusDxe scans all MaxTarget * MaxLun possible devices.
This can take unnecessarily long for large number of targets.
To deal with this, VirtioScsiDxe has defined PCDs to limit the
MaxTarget & MaxLun to desired values which gives sufficient
performance. It is very important in virtio-scsi as it can have
very big MaxTarget & MaxLun.
Even though a common PVSCSI device has a default MaxTarget=64 and
MaxLun=0, we implement similar mechanism as virtio-scsi for completeness.
This may be useful in the future when PVSCSI will have bigger values
for MaxTarget and MaxLun.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2567
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Liran Alon <liran.alon@oracle.com>
Message-Id: <20200328200100.60786-7-liran.alon@oracle.com>
Reviewed-by: Nikita Leshenko <nikita.leshchenko@oracle.com>