target-i386: add a generic x86 nmi handler

Instead of having x86 ifdefs in core nmi code, this
change adds a arch specific handler that the nmi common
code can call.

Signed-off-by: Bandan Das <bsd@redhat.com>
Message-Id: <1463761717-26558-2-git-send-email-bsd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>