dmidecode: Handle missing values

Make all hwInfo fields in API optional, and change property retrieval so
it cannot fail, also, get rid of all keys that contain no actual
information.

The engine will assume that all the missing keys are not available and
report/display an according message (rather than sending 'Not
Specified')

Change-Id: I979518e1d0c0c882fe98fd5aee43c0d50ab17e14
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1089393
Signed-off-by: Yaniv Bronhaim <ybronhei@redhat.com>
Signed-off-by: Dima Kuznetsov <dkuznets@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26942
Reviewed-by: Dan Kenigsberg <danken@redhat.com>