kvm tools: allow arch's to provide their own command-line options

Currently, only x86 has architecture command-line options (for setting
the BIOS video mode) however this is likely to become more common in the
future.

This patch adds some simple macros and a struct definition to allow
architectures to augment the command-line options with private
definitions. The BIOS video mode option (--vidmode) is also migrated to
the new framework.

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