kvm: Reallocate dirty_bmap when we change a slot

kvm: Reallocate dirty_bmap when we change a slot

kvm_set_phys_mem can be called to reallocate a slot by something the
guest does (e.g. writing to PAM and other chipset registers).
This can happen in the middle of a migration, and if we're unlucky
it can now happen between the split 'sync' and 'clear'; the clear
asserts if there's no bmap to clear.   Recreate the bmap whenever
we change the slot, keeping the clear path happy.

Typically this is triggered by the guest rebooting during a migrate.

Corresponds to:
https://bugzilla.redhat.com/show_bug.cgi?id=1772774
https://bugzilla.redhat.com/show_bug.cgi?id=1771032

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
(cherry picked from commit 9b3a31c745b61758aaa5466a3a9fc0526d409188)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>