OvmfPkg: get boot order from QEMU

OvmfPkg: get boot order from QEMU

Set the boot order based on configuration retrieved from QEMU.

Attempt to retrieve the "bootorder" fw_cfg file from QEMU. Translate the
OpenFirmware device paths therein to UEFI device path fragments. Match the
translated fragments against the enumerated BootOptionList, and rewrite
the BootOrder NvVar so that it corresponds to the order described in
fw_cfg.

The user is expected to configure working boot options first.

Tested via virt-manager's boot order widget.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@13683 6f19259b-4bc3-4df7-8a09-765794883524