We document that calling any public API wipes out all prior
libvirt errors in the same thread; but weren't obeying this
style in a few functions.

There are a couple of nested uses of virConnectRef (in lxc
and qemu reboot paths), but they should not be affected by
this change in semantics since there should not be any
previous error getting nuked (a later patch will clean up
the nested calls, along with abuse of virConnectClose on
cleanup paths which DOES nuke errors).

* src/libvirt.c (virGetVersion, virConnectRef, virDomainRef)
(virDomainGetSecurityLabel, virDomainGetSecurityLabelList)
(virDomainSetMetadata, virDomainGetMetadata)
(virNodeGetSecurityModel, virNetworkRef, virInterfaceRef)
(virStoragePoolRef, virStorageVolRef, virNodeDeviceGetName)
(virNodeDeviceRef, virSecretRef, virStreamRef, virNWFilterRef)
(virDomainSnapshotRef): Reset error on entrance.
(do_open): Drop redundant error reset.
* src/libvirt-qemu.c (virDomainQemuAgentCommand): Likewise.
* src/libvirt-lxc.c (virDomainLxcEnterNamespace)
(virDomainLxcEnterSecurityLabel): Likewise.
Signed-off-by: NEric Blake <eblake@redhat.com>