maint: improve error condition style in public API

maint: improve error condition style in public API

While auditing error messages in libvirt.c, I found a couple
instances that had not been converted to modern error styles,
and a few places that failed to dispatch the error through
the known-good connection.

* src/libvirt.c (virDomainPinEmulator, virDomainGetDiskErrors)
(virDomainSendKey, virDomainGetSecurityLabelList)
(virDomainGetEmulatorPinInfo): Use typical error reporting.
(virConnectGetCPUModelNames, virConnectRegisterCloseCallback)
(virConnectUnregisterCloseCallback, virDomainGetUUID): Report
error through connection.

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