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>
I advise you to use vmxml.sync() to avoid leaving undefined after failed definition.