hw/i386: Build-time assertion on pc/q35 reset register being identical.

This adds a clarifying comment and build time assert to the FADT reset register field initialisation: the reset register is the same on both machine types.

Signed-off-by: Phil Dennis-Jordan <phil@philjordan.eu>
Message-Id: <1489558827-28971-3-git-send-email-phil@philjordan.eu>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>