All host_msr_state accesses are solely on the owning CPU, and all
guest_msr_state ones solely when the vCPU is current or being switched
to. This, btw, is also in line with the use of find_first_set_bit()
(which would be bogus if ->flags could get updated behind its back).

Signed-off-by: Jan Beulich <jbeulich@suse.com>