kvm-all.c: max_cpus should not exceed KVM vcpu limit

maxcpus, which specifies the maximum number of hotpluggable CPUs,
should not exceed KVM's vcpu limit.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
[Reword message. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
