kvm tools: Limit CPU count by KVM_CAP_NR_VCPUS

This patch limits the number of CPUs to KVM_CAP_NR_VCPUS when user specifies
more CPUs with the "--cpus=N" command line option than what the in-kernel KVM
is able to handle.

Cc: Asias He <asias.hejun@gmail.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Prasad Joshi <prasadjoshi124@gmail.com>
Cc: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>