virt.network_define allow adding IP configuration

If using virt.network_define with nat network type, then libvirt
complains about missing IP configuration. Allow setting it in both the
virt.network_define module and the virt.network_running state.

Add virt.pool_deleted state

The new virt.pool_deleted state takes care of removing a virtual storage
pool and optionally the contained volumes.

Fix virt._gen_pool_xml when all source parameters are None

When all the source_* parameters of _gen_pool_xml are None, no <source>
element should be generated. Instead we had the following:

    <source><path dir="None"/></source>

Rather than filling a source structure with empty members, directly pass
a None for the source as expected by the jinja template.

Add virt.pool_update function

Add a function that allows changing a virtual storage pool. By using the
test=True parameter, this function can be used to check if a change
would be done.

virt.pool_running state improvements

With this commit the virt.pool_running state is capable to update an
existing pool before ensuring it is running.

This also adds support for the test parameter.

Fix virt.pool_define documentation

virt.pool_define example are mentioning a uuid property while this one
should be named value. The code was right, but not the doc.

Add source_initiator parameter in virt.pool_define

In order to define iscsi-direct virtual storage pools, the use needs to
be able to provide the initiator IQN. Add a parameter for it in:
  * virt.pool_define module function
  * virt.pool_update module function
  * virt.pool_running state

virt pool secret have no type attribute

According to libvirt schemas and doc only the volume secrets have a
type.

virt: create and update pool secret's password for the user

Libvirt stores RBD and iSCSI passwords in secrets. Add a password field
in the source_auth parameter of virt.pool_define and virt.pool_update to
set the password value rather than reuse an already defined libvirt
secret.

virt.pool_define: remove potential leading / in CIFS source path

libvirt needs the CIFS source path not to start with a /. Let's remove
them since this could be a common user mistake.

Add virt.pool_capabilities function

Not all storage backends are supported for a given libvirt hypervisor. For the user
to know what is supported and what is not expose the recently added
libvirt function providing pool capabilities.

For older libvirt version, let's craft data that are reasonable by
adding a computed flag to warn these data may not be 100% accurate.