The virDomainSnapshotDefFormat calls into virDomainDefFormat,
so should be providing a non-NULL virCapsPtr instance. On the
qemu driver we change qemuDomainSnapshotWriteMetadata to also
include caps since it calls virDomainSnapshotDefFormat.
Signed-off-by: NJoao Martins <joao.m.martins@oracle.com>