vl: fix direct firmware directories leak

Note that data_dir[] will now point to allocated strings.

Fixes:
Direct leak of 16 byte(s) in 1 object(s) allocated from:
    #0 0x7f1448181850 in malloc (/lib64/libasan.so.4+0xde850)
    #1 0x7f1446ed8f0c in g_malloc ../glib/gmem.c:94
    #2 0x7f1446ed91cf in g_malloc_n ../glib/gmem.c:331
    #3 0x7f1446ef739a in g_strsplit ../glib/gstrfuncs.c:2364
    #4 0x55cf276439d7 in main /home/elmarco/src/qq/vl.c:4311
    #5 0x7f143dfad039 in __libc_start_main (/lib64/libc.so.6+0x21039)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180104160523.22995-10-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>