Symantec recommends to install SEP on both the Windows machines (Guest and Host ) in order to protect the machines.
Each machine will be treated as separate machine by SEPM and will consume one license each.
Example : If a Windows Host machine (Hyper-V , VMware Workstation) ,hosts another Windows guest machine then we do have to install SEP on both the machines as both of them are treated as separate machines by SEPM. Both the machines will consume one SEP license each.
(i.e. Two Licenses as per above example )