Introduce "xen-save-devices-state"

Introduce "xen-save-devices-state"

- add an "is_ram" flag to SaveStateEntry;

- register_savevm_live sets is_ram for live_savevm devices;

- introduce a "xen-save-devices-state" QAPI command that can be used to save
the state of all devices, but not the RAM or the block devices of the
VM.

Changes in v8:

- rename save-devices-state to xen-save-devices-state.

Changes in v7:

- rename save_devices to save-devices-state.

Changes in v6:

- remove the is_ram parameter from register_savevm_live and sets is_ram
if the device is a live_savevm device;

- introduce save_devices as a QAPI command, write a better description
for it;

- fix CODING_STYLE;

- introduce a new doc to explain the save format used by save_devices.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Acked-by: Luiz Capitulino <lcapitulino@redhat.com>