migration: fix incorrect bitmap size calculation

migration: fix incorrect bitmap size calculation

The calculation on size of received bitmap is incorrect for postcopy
recovery.  Here we wanted to let the size to cover all the valid bits in
the bitmap, we should use DIV_ROUND_UP() instead of a division.

For example, a RAMBlock with size=4K (which contains only one single 4K
page) will have nbits=1, then nbits/8=0, then the real bitmap won't be
sent to source at all.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180710091902.28780-4-peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>