migrate: Fix cpu-throttle-increment regression in HMP

Commit 69ef1f36 accidentally broke migrate_set_parameter's ability
to set the cpu-throttle-increment to anything other than the
default, because it forgot to parse the user's string into an
integer.

CC: qemu-stable@nongnu.org
Signed-off-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NJuan Quintela <quintela@redhat.com>
Signed-off-by: NJuan Quintela <quintela@redhat.com>
(cherry picked from commit bb2b777c)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

hmp.c

@@ -1284,6 +1284,7 @@ void hmp_migrate_set_parameter(Monitor *mon, const QDict *qdict)
                 break;
             case MIGRATION_PARAMETER_CPU_THROTTLE_INCREMENT:
                 has_cpu_throttle_increment = true;
+                use_int_value = true;
                 break;
             case MIGRATION_PARAMETER_TLS_CREDS:
                 has_tls_creds = true;