backend: check image status before ExportRepoImage

Before running the ExportRepoImage command we should also check if the
image is locked or illegal and in such case prevent the export.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1024883
Change-Id: Ib0957a05db4a0fdcddf56a63b0a3da0b4d27db6d
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>