qemu: migration: Avoid use of 'nmigration_disks'

qemu: migration: Avoid use of 'nmigration_disks'

'migration_disks' is a NULL-terminated string list, so the code can be
converted to either iterate the string-list, use existing accessors or
check the presence of the pointers instead of checking the count.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>