Commit 51b0d834 changed error handling to report file name in error
message but forgot to move freeing it after usage. Noticed by Coverity.

Fixes: CID 1394217
Reported-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NBALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>