fix savevm command without id or tag

fix savevm command without id or tag

savevm without id or tag segfaults in:

(gdb) bt
#0  0x00007f600a83bf8a in __strcmp_sse42 () from /lib64/libc.so.6
#1  0x00000000004745b6 in bdrv_snapshot_find (bs=<value optimized out>,
    sn_info=0x7fff996be280, name=0x0) at savevm.c:1631
#2  0x0000000000475c80 in del_existing_snapshots (name=<value optimized out>,
    mon=<value optimized out>) at savevm.c:1654
#3  do_savevm (name=<value optimized out>, mon=<value optimized out>)

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>