util: virerror: Introduce virGetLastError{Code,Domain} public APIs

util: virerror: Introduce virGetLastError{Code,Domain} public APIs

Many places in the code call virGetLastError() just to check the
raised error code, or domain. However virGetLastError() can return
NULL, so the code has to check for that first. This patch therefore
introduces virGetLasError{Code,Domain} functions which always return a
valid error code or domain respectively, thus dropping the need to
perform any checks on the error object.

Signed-off-by: Ramy Elkest <ramyelkest@gmail.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>