Allow XBZRLE decoding without enabling the capability

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: Orit Wasserman <owasserm@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

Signed-off-by: Juan Quintela <quintela@redhat.com>