KVM: x86: Raise the hard VCPU count limit

The patch raises the hard limit of VCPU count to 254.

This will allow developers to easily work on scalability
and will allow users to test high VCPU setups easily without
patching the kernel.

To prevent possible issues with current setups, KVM_CAP_NR_VCPUS
now returns the recommended VCPU limit (which is still 64) - this
should be a safe value for everybody, while a new KVM_CAP_MAX_VCPUS
returns the hard limit which is now 254.

Cc: Avi Kivity <avi@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Pekka Enberg <penberg@kernel.org>
Suggested-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>