kvm tools: arm: make .dtb dumping a command-line option

It can sometimes be useful to dump the .dtb file generated by kvmtool
when debugging a guest. Currently, this is achieved by rebuilding the
tool and changing some #defines, which is fairly clumsy to use.

This patch adds a new command-line option for ARM, allowing the dtb
to be dumped to a named file at runtime.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>