vl.c: Fix Coverity complaining for vmstate_dump_file

vl.c: Fix Coverity complaining for vmstate_dump_file

commit abfd9ce3(migration: dump vmstate info as a json
file for static analysis) introduce a new command,
'-dump-vmstate', that takes a filename
as an argument.  When executed, QEMU will dump the vmstate information
for the machine type it's invoked with to the file, and quit.

However, only one instance of the -dump-vmstate option is supported.
If more were given, the vmstate_dump_file variable would be overwritten.

This fix also helps silence a Coverity error.

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>