Revert all previous error log priority hacks

Revert all previous error log priority hacks

This reverts the additions in commit

  abff683f78ffd01df5005fb7a457c0b38b8eb6e1

taking us back to state where all errors are fully logged
in both libvirtd and normal clients.

THe intent was to stop VIR_ERR_NO_DOMAIN (No such domain
with UUID XXXX) messages from client apps polluting syslog
The change affected all error codes, but more seriously,
it also impacted errors from internal libvirtd infrastructure
For example guest autostart no longer logged errors. The
libvirtd network code no longer logged some errors. This
makes debugging incredibly hard

* daemon/libvirtd.c: Remove error log priority filter
* src/util/virterror.c, src/util/virterror_internal.h: Remove
  callback for overriding log priority