maint: improve VIR_ERR_INVALID_CONN usage

maint: improve VIR_ERR_INVALID_CONN usage

The datatype.c object checks could result in a message like:

error: invalid connection pointer in no connection

This consolidates all clients of this message to have uniform contents:

error: invalid connection pointer in someFunc

Note that virCheckConnectReturn raises an error immediately; in
datatypes.c, where we don't need to raise the error (but instead
just leave it in the thread-local setting), we use
virCheckConnectGoto and the cleanup label instead.  Then, for
consistency in that file, all subsequent error messages are
touched to also use the cleanup error label.

* src/datatypes.h (virCheckConnectReturn)
(virCheckConnectGoto): New macros.
* src/datatypes.c: Use new macro.
* src/libvirt-qemu.c (virDomainQemuAttach): Likewise.
(virLibConnError): Delete unused macro.
* src/libvirt-lxc.c (virLibConnError): Likewise.
* src/libvirt.c: Use new macro throughout.
* docs/api_extension.html.in: Modernize documentation.

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