hw/ppc: Implement -dtb support for PowerNV

hw/ppc: Implement -dtb support for PowerNV

Currently any device tree passed with -dtb option in QEMU, was ignored
by the PowerNV code.

Read and pass the passed -dtb to the kernel, thus enabling easier
debugging with custom DTBs.

The existing behaviour when -dtb is 'not' passed, is preserved as-is.

But when a '-dtb' is passed, it completely overrides any dtb nodes or
changes QEMU might have done, such as '-append' arguments to the kernel
(which are mentioned in /chosen/bootargs in the dtb), hence add warning
when -dtb is being used

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Aditya Gupta <adityag@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>