qdev: add command line option to set global defaults for properties.

This patch adds infrastructure and command line option for setting
global defaults for device properties, i.e. you can for example use

  -global virtio-blk-pci.vectors=0

to turn off msi by default for all virtio block devices.  The config
file syntax is:

[global]
  driver = "virtio-blk-pci"
  property = "vectors"
  value = "0"

This can also be used to set properties for devices which are not
created via -device but implicitly via machine init, i.e.

  -global isa-fdc,driveA=<name>

This patch uses the mechanism which configures properties for the
compatibility machine types (pc-0.10 & friends).  The command line
takes precedence over the machine type values.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>