Add PIIX4 properties to control PM system states.

Add PIIX4 properties to control PM system states.

This patch adds two things. First it allows QEMU to distinguish between
regular powerdown and S4 powerdown. Later separate QMP notification will
be added for S4 powerdown. Second it allows S3/S4 states to be disabled
from QEMU command line. Some guests known to be broken with regards to
power management, but allow to use it anyway. Using new properties
management will be able to disable S3/S4 for such guests.

Supported system state are passed to a firmware using new fw_cfg file.
The file contains  6 byte array. Each byte represents one system
state. If byte at offset X has its MSB set it means that system state
X is supported and to enter it guest should use the value from lowest 3
bits.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>