Fixed set_hosts for handling correctly -H option

As stated in the method documentation each option set together
is treated as an intersection.
Previously -H caused an error when a host list was supplied due
to a parameter passed with the wrong type to get_hypervisor_data.

Change-Id: Ib9a99fc1e9bd49fbdcfcbdcb18dfddf6cd7e80e6
Bug-Url: https://bugzilla.redhat.com/918020
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>