tests/qemu-iotests: adding savevm/loadvm with postcopy flag test

This patch implements a test case for the scenario that was failing
prior to the patch "migration/ram.c: do not set 'postcopy_running' in
POSTCOPY_INCOMING_END", commit acab30b8.

This new test file 201 was derived from the test file 181 authored
by Kevin Wolf.

CC: Kevin Wolf <kwolf@redhat.com>
CC: Max Reitz <mreitz@redhat.com>
CC: Cleber Rosa <crosa@redhat.com>
Signed-off-by: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>