Remove the KVM_MIGRATION_PORT configure.ac param

Since this is easily configurable at run-time, we remove the
configure-time parameter. If anyone is building custom packages, then
the default can be tweaked by a one-line patch to constants.py.

Note that this also fixes the type of parameter, the default from
_autoconf.py is a string parameter. Shouldn't matter except if a cluster
run code between commit 78411c60 and this one.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>