virt: fix signature of set_admin_password method

The compute manager passes (instance, new_pass) to the
set_admin_password method, and the XenAPI driver impl
expects this pair of parameters. The ComputeManager
class, however, declares that it takes (instance, context,
new_pass=None). It makes no sense to default the new
password to None, when the only caller always passes
it, and the context parameter is never passed by the
caller either.

Partial-bug: #1333219
Change-Id: If656da4ef54a0425b9f50ef03af044b803b1afae
Merge "virt: fix signature of set_admin_password method"