migration: fix pfd leak

migration: fix pfd leak

Fix leak spotted by ASAN:

Direct leak of 16 byte(s) in 1 object(s) allocated from:
    #0 0x7fe1abb80a38 in __interceptor_calloc (/lib64/libasan.so.4+0xdea38)
    #1 0x7fe1aaf1bf75 in g_malloc0 ../glib/gmem.c:124
    #2 0x7fe1aaf1c249 in g_malloc0_n ../glib/gmem.c:355
    #3 0x55f4841cfaa9 in postcopy_ram_fault_thread /home/elmarco/src/qemu/migration/postcopy-ram.c:596
    #4 0x55f48479447b in qemu_thread_start /home/elmarco/src/qemu/util/qemu-thread-posix.c:504
    #5 0x7fe1a043550a in start_thread (/lib64/libpthread.so.0+0x750a)

Regression introduced with commit 00fa4fc85b00f1a8a810068d158a7a66e88658eb.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180321113644.21899-1-marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>