kvm: x86: Increase KVM_SOFT_MAX_VCPUS to 710

Support for 710 VCPUs was tested by Red Hat since RHEL-8.4,
so increase KVM_SOFT_MAX_VCPUS to 710.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20210903211600.2002377-4-ehabkost@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>