Fix Software Update rebase issue.

V1 -> V2:

SoftwareUpdate() executes an exception if there's no pkg manager in the host, so
I changed previous patch to use try/except while instantiating PackagesUpdateModel()

V1:

During the last merge/rebase to apply Software Update feature to the code,
an important line was removed. This fix is adding it again and enabling the
code to list all host's package updates.

Signed-off-by: Paulo Vital <pvital@linux.vnet.ibm.com>
Signed-off-by: Aline Manera <alinefm@br.ibm.com>