webadmin: Change valid range for VmPool.MaxAssignedVmsPerUser

Enable VmPools to have MaxAssignedVmsPerUser any value in
the range 1..Integer.INT_MAX.

This simplifies the validation and also enables to set this
property during the creation of the VmPool which was not
possible since the valid range was 1..AssignedVmsCount.

Change-Id: Ibf343108620e3625696711750f7e82416b7050e0
Signed-off-by: Martin Betak <mbetak@redhat.com>