migration/postcopy: make PostcopyDiscardState a static variable

migration/postcopy: make PostcopyDiscardState a static variable

In postcopy-ram.c, we provide three functions to discard certain
RAMBlock range:

  * postcopy_discard_send_init()
  * postcopy_discard_send_range()
  * postcopy_discard_send_finish()

Currently, we allocate/deallocate PostcopyDiscardState for each RAMBlock
on sending discard information to destination. This is not necessary and
the same data area could be reused for each RAMBlock.

This patch defines PostcopyDiscardState a static variable. By doing so:

  1) avoid memory allocation and deallocation to the system
  2) avoid potential failure of memory allocation
  3) hide some details for their users

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Message-Id: <20190724010721.2146-1-richardw.yang@linux.intel.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>