Before this fix we couldn't load a guest from
XBZRLE compressed file.

For example:
The user activated the XBZRLE capability
The user run migrate -d "exec:gzip -c > vm.gz"
The user won't be able to load vm.gz and get an error.
Signed-off-by: NOrit Wasserman <owasserm@redhat.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NJuan Quintela <quintela@redhat.com>