Today, it's necessary to specify the protocol you want to use
when dumping the guest memory, for example:

 (qemu) dump-guest-memory file:/tmp/guest-memory

This has a few issues:

 1. It's cumbersome to type
 2. We loose file path autocompletion
 3. Being able to specify fd:X in HMP makes little sense for humans

Because of these reasons, hardcode the 'protocol' argument to
'file:' in HMP.
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>