fw_cfg: Split fw_cfg_keys.h off fw_cfg.h

Much of fw_cfg.h's contents is #ifndef NO_QEMU_PROTOS.  This lets a
few places include it without satisfying the dependencies of the
suppressed code.  If you somehow include it with NO_QEMU_PROTOS, any
future includes are ignored.  Unnecessarily unclean.

Move the stuff not under NO_QEMU_PROTOS into its own header
fw_cfg_keys.h, and include it as appropriate.  Tidy up the moved code
to please checkpatch.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
