monitor: Use argument type 'b' for migrate_set_speed

Before, it used type 's', which strips quotes and interprets escapes,
and is quite inappropriate for QMP.

Negative arguments are no flushed to zero.  Before, they were cast to
uint32_t, which wrecked the sign.

Ridiculously large arguments including infinities are now rejected.
Before, they were interpreted as zero.  Same for NaN.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
