maint: improve VIR_ERR_INVALID_DOMAIN_SNAPSHOT usage
    
    The existing check of domain snapshots validated that they
    point to a domain, but did not validate that the domain
    points to a connection, even though any errors blindly assume
    the connection is valid.  On the other hand, as mentioned in
    commit 6e130ddc, any valid domain is already tied to a valid
    connection, and VIR_IS_SNAPSHOT vs. VIR_IS_DOMAIN_SNAPSHOT
    makes no real difference; it's best to just validate the chain
    of all three.  For consistency with previous patches, continue
    the trend of using a common macro.  For now, we don't need
    virCheckDomainSnapshotGoto().
    
    * src/datatypes.h (virCheckDomainSnapshotReturn): New macro.
    (VIR_IS_SNAPSHOT, VIR_IS_DOMAIN_SNAPSHOT):
    Drop unused macros.
    * src/libvirt.c: Use macro throughout.
    (virLibDomainSnapshotError): Drop unused macro.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>