Use g_auto for virURI almost everywhere

Use g_auto for virURI almost everywhere

Convert all the cases where we can unconditionally free
the virURI at the end of scope.

In libxlDomainMigrationDstPrepare, uri is only filled
if uri_in was present, so moving the virURIFree out of
the condition is safe.

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>