maint: move debug statements first in public API

maint: move debug statements first in public API

Most of our public APIs emit a debug log on entry, prior to anything
else.  There were a few exceptions where obvious failures were not
logged, so fix those.  When moving a debug earlier, this patch also
makes sure to avoid any NULL dereference during the log (the APIs
are supposed to gracefully fail if the user passes NULL for the object).

However, do NOT use VIR_DEBUG prior to virInitialize, since setting
up the error reporting can change where VIR_DEBUG output would be
routed.  Instead add documentation to virGlobalInit, virInitialize,
and virGetVersion that better explains initialization.

* src/libvirt.c (virGetVersion, virConnectRef, virDomainRef)
(virNetworkRef, virInterfaceRef, virStoragePoolRef)
(virStorageVolRef, virNodeDeviceRef, virSecretRef, virStreamRef)
(virNWFilterRef, virDomainSnapshotRef): Debug on function entry.
* src/libvirt-lxc.c (virDomainLxcEnterNamespace)
(virDomainLxcEnterSecurityLabel): Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>