restapi: Throw exception if setting lower VM pool size

Report, if the user tries to lower the number
of VMs in the VM pool.

Change-Id: Ibe4ad50e6962159df5891be4653bc05eb3dfd237
Bug-Url: https://bugzilla.redhat.com/950049
Signed-off-by: Libor Spevak <lspevak@redhat.com>