KVM: document target of capability enablement

Capabilities can be enabled on a vcpu or (since recently) on a vm. Document
this and note for the existing capabilites whether they are per-vcpu or
per-vm.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>