doc: kvm: Fix return description of KVM_SET_MSRS

Userspace can use ioctl KVM_SET_MSRS to update a set of MSRs of guest.
This ioctl set specified MSRs one by one. If it fails to set an MSR,
e.g., due to setting reserved bits, the MSR is not supported/emulated by
KVM, etc..., it stops processing the MSR list and returns the number of
MSRs have been set successfully.

Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>