ipmi: Use proper struct reference for BT vmstate

ipmi: Use proper struct reference for BT vmstate

The vmstate for isa_ipmi_bt was referencing into the bt structure,
instead create a bt structure separate and use that.

The version 1 of the BT transfer was fairly broken, if a migration
occured during an IPMI operation, it is likely the migration would
be corrupted because I misunderstood the VMSTATE_VBUFFER_UINT32()
handling, I thought it handled transferring the length field,
too.  So I just remove support for that.  I doubt anyone is using
it at this point.

This also removes the transfer of use_irq, since that should come
from configuration.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <1534798644-13587-1-git-send-email-minyard@acm.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>