dump: add 'query-dump-guest-memory-capability' command

dump: add 'query-dump-guest-memory-capability' command

'query-dump-guest-memory-capability' is used to query the available formats for
'dump-guest-memory'. The output of the command will be like:

-> { "execute": "query-dump-guest-memory-capability" }
<- { "return": { "formats":
                    ["elf", "kdump-zlib", "kdump-lzo", "kdump-snappy"] }

Signed-off-by: Qiao Nuohan <qiaonuohan@cn.fujitsu.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>