vm: fix odd migration progress reporting

migration progress should not start from 100,
otherwise the user will see smething like:

progress: 100 -> 0% ... X% ... 100%

A real-word scenario that shows the need for this patch is
the following:
If during the the initialization of migration we experience
some delay, for example DNS resolve issues, libvirt returns 0
for all the progress values, and that incorrectly triggered
the old shortcut, leading to an incorrect initial progress
value of 100.

Bug-Url: https://bugzilla.redhat.com/1104670
Change-Id: Ib9d72b0b8b956d79a7f69df61f4b45048dc9443e
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27465
Tested-by: Tomas Jelinek <tjelinek@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28363