machine: package all init arguments into a QemuOpts (v2)

machine: package all init arguments into a QemuOpts (v2)

This patch creates a QemuOpts structure and stores all of the machine init
arguments in that structure.  It introduces a temporary list of QemuOptDescs
in vl.c such that the current common options can be validated.

The long term vision is that that list becomes a #define and that each machine
can optionally provide it's own QemuOptDescs list using the common options as
a base.  This enables per-machine options.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
--
v1 -> v2:
  - cpu_model -> cpu