vm_xml: let delete device can also by device tag

In some situations, we don't care the device xml, so after this patch,
we can avoid generate device xml by set 'by_tag=True'.

Signed-off-by: Yanbing Du <ydu@redhat.com>
vm_xml: Add memtune XML support and add 'unit' attribute for memory element

Signed-off-by: Yanbing Du <ydu@redhat.com>
Merge pull request #1848 from will-Do/memtune_xml

vm_xml: Add memtune XML support
Merge pull request #1852 from will-Do/del_device

vm_xml: let delete device can also by device tag
utils_test.libvirt: Add 3 vm configuration functions

Add 3 vm configuration functions for manipulate domain state, guest
agent setup and domain disk replacement, which offer pre-test domain
configuration for testing.

The set_vm_disk function is new here, which take a lot of usage with
functions exist in current module, e.g. setup iscsi, nfs, gluster for
storage backend.

Disk replacement is for cover different disk types as supported in
domain xml, including file type(local, nfs), network type(gluster,
iscsi), block type(use connected iscsi block disk). This is different
than setting up pool first and use the vol in the pool, which covered
in pool related testing.

Signed-off-by: Wayne Sun <gsun@redhat.com>