monitor: print the invalid char in error message

It's more friendly to print which char is invalid to user, especially
when user tries to input a float value and expect the monitor to round
it to int. Since we don't round float number when we look for a integer,
telling which char is invalid is less confusing.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
