efi_loader: device path for SATA devices

Provide device path nodes for SATA devices.

This avoids creation of two handles with the same device path indicating
our root node.

This is what the device paths for a SATA drive with four partitions could
like:

/VenHw(..)/Sata(0x0,0xffff,0x0)
/VenHw(..)/Sata(0x0,0xffff,0x0)/HD(1,MBR,0x81ea591f,0x800,0x63ff830)
/VenHw(..)/Sata(0x0,0xffff,0x0)/HD(2,MBR,0x81ea591f,0x6400800,0x9ff830)
/VenHw(..)/Sata(0x0,0xffff,0x0)/HD(3,MBR,0x81ea591f,0x6e00800,0x16ef2ab0)
/VenHw(..)/Sata(0x0,0xffff,0x0)/HD(4,MBR,0x81ea591f,0x1dcf3800,0x1dcedab0)

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>