monitor/hmp: add support for flag argument with value

monitor/hmp: add support for flag argument with value

Adds support for the "-xs" parameter type, where "-x" denotes a flag
name and the "s" suffix indicates that this flag is supposed to take
an arbitrary string parameter.

These parameters are always optional, the entry in the qdict will be
omitted if the flag is not given.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
[FE: fixed typo pointed out by Eric Blake
     use s instead of V to indicate string parameter]
Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>
Message-Id: <20220225084949.35746-2-f.ebner@proxmox.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>