softmmu/dirtylimit: Add parameter check for hmp "set_vcpu_dirty_limit"

softmmu/dirtylimit: Add parameter check for hmp "set_vcpu_dirty_limit"

dirty_rate paraemter of hmp command "set_vcpu_dirty_limit" is invalid
if less than 0, so add parameter check for it.

Note that this patch also delete the unsolicited help message and
clean up the code.

Signed-off-by: Hyman Huang(黄勇) <yong.huang@smartx.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <168618975839.6361.17407633874747688653-1@git.sr.ht>
Signed-off-by: Juan Quintela <quintela@redhat.com>