fw_cfg: factor out initialization of FW_CFG_ID (rev. number)

The fw_cfg documentation says this of the revision key (0x0001, FW_CFG_ID):

> A 32-bit little-endian unsigned int, this item is used as an interface
> revision number, and is currently set to 1 by all QEMU architectures
> which expose a fw_cfg device.

arm/virt doesn't.  It could be argued that that's an error in
"hw/arm/virt.c"; on the other hand, all of the other fw_cfg providing
boards set the interface version to 1 manually, despite the device
coming from the same, shared implementation. Therefore, instead of
adding

    fw_cfg_add_i32(fw_cfg, FW_CFG_ID, 1);

to arm/virt, consolidate all such existing calls in the fw_cfg
initialization code.

Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
Message-Id: <1426789244-26318-1-git-send-email-somlo@cmu.edu>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
