Add 'zvm' to the list of known hypervisor types

zvm is a hypervisor that used for IBM s390 arch and has
many years history on virtualization.
you can refer to http://www.vm.ibm.com/ for more info on it.

According to danpb's comments on
https://review.openstack.org/#/c/122200/:
FYI the list of hvtype constants is intended to define a global set of
all theoretically supported hypervisors. ie it is fine to add more
constants for hypervisors, even if Nova doesn't have a driver in-tree for it.

even if zvm is not in the nova tree now, we are trying to enhance the
code and other related stuffs such as CI test to make it acceptable to
community later. Also, zvm driver code is not in stackforge now since
we are trying to fit other criterias in order to make community accept it and
put into nova tree.

This patch simply adds zvm to the list of known hypervisor types which are
validated during scheduling given commit a4fd236.

Change-Id: I1f5b5bd2a8f082d2e945560ae82593dfd70f2a22

nova/compute/hvtype.py

@@ -25,8 +25,8 @@
 from nova import exception
 
 
-# This list is taken from libvirt, with OpenStack
-# specific 'baremetal' & 'fake' then added in.
+# This list is all known hypervisors
+# even if not currently supported by OpenStack.
 BAREMETAL = "baremetal"
 BHYVE = "bhyve"
 FAKE = "fake"
@@ -43,6 +43,7 @@
 VBOX = "vbox"
 VMWARE = "vmware"
 XEN = "xen"
+ZVM = "zvm"
 
 ALL = (
     BAREMETAL,
@@ -61,6 +62,7 @@
     VBOX,
     VMWARE,
     XEN,
+    ZVM,
 )