efi_loader: device_path: support Sandbox's "host" devices

Sandbox's "host" devices are currently described as UCLASS_ROOT udevice
with DEV_IF_HOST block device. As the current implementation of
efi_device_path doesn't support such a type, any "host" device
on sandbox cannot be seen as a distinct object.

For example,
  => host bind 0 /foo/disk.img

  => efi devices
  Scanning disk host0...
  Found 1 disks
  Device           Device Path
  ================ ====================
  0000000015c19970 /VenHw(e61d73b9-a384-4acc-aeab-82e828f3628b)
  0000000015c19d70 /VenHw(e61d73b9-a384-4acc-aeab-82e828f3628b)

  => efi dh
  Handle           Protocols
  ================ ====================
  0000000015c19970 Device Path, Device Path To Text, Device Path Utilities, Unicode Collation 2, HII String, HII Database, HII Config Routing
  0000000015c19ba0 Driver Binding
  0000000015c19c10 Simple Text Output
  0000000015c19c80 Simple Text Input, Simple Text Input Ex
  0000000015c19d70 Block IO, Device Path, Simple File System

As you can see here, efi_root (0x0000000015c19970) and host0 device
(0x0000000015c19d70) have the same representation of device path.

This is not only inconvenient, but also confusing since two different
efi objects are associated with the same device path and
efi_dp_find_obj() will possibly return a wrong result.

Solution:
Each "host" device should be given an additional device path node
of "vendor device path" to make it distinguishable.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>