hw/pc_sysfw: support system flash memory with pflash

hw/pc_sysfw: support system flash memory with pflash

Flash can be enabled by calling pc_system_firmware_init
with the system_flash_enabled parameter being non-zero.
If system_flash_enabled is zero, then the older qemu
rom creation method will be used.

If flash is enabled and a pflash image is found, then
it is used for the system firmware image.

If flash is enabled and a pflash image is not initially
found, then a read-only pflash device is created using
the -bios filename.

KVM cannot execute from a pflash region currently.
Therefore, when KVM is enabled, the old rom based
initialization method is used.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>