KVM: use KVM_CAP_MAX_VCPU_ID

KVM: use KVM_CAP_MAX_VCPU_ID

As stated in linux/Documentation/virtual/kvm/api.txt:

The maximum possible value for max_vcpu_id can be retrieved using the
KVM_CAP_MAX_VCPU_ID of the KVM_CHECK_EXTENSION ioctl() at run-time.

If the KVM_CAP_MAX_VCPU_ID does not exist, you should assume that
max_vcpu_id is the same as the value returned from KVM_CAP_MAX_VCPUS.

Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Message-Id: <146424974323.5666.5471538288045048119.stgit@bahia.huguette.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>