XenAPI: Use GUEST_PASSWORD when installing DomU

We know what the GUEST_PASSWORD will be, so set it during DomU
installation; therefore if the installation fails for any reason
we have a correct GUEST_PASSWORD set rather than the obscure and
undocumented 'r00tme'

Change-Id: I684646a74f2cede43667a40556e247955e042ae4
Merge "XenAPI: Use GUEST_PASSWORD when installing DomU"