OvmfPkg/LegacyBbs: Add boot entries for VirtIO and NVME devices

OvmfPkg/LegacyBbs: Add boot entries for VirtIO and NVME devices

Iterate over the available block devices in much the same way as
BdsLibEnumerateAllBootOption() does, but limiting to those devices
which are PCI-backed, which can be represented in the BbsTable.

One day we might need to extend the BbsTable to allow us to distinguish
between different NVMe namespaces on a device.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20190626113742.819933-3-dwmw2@infradead.org>