session: prefix proxy lookup errors

Proxy errors are already reported with G_IO_ERROR_PROXY messages,
however proxy name lookup error can be more detailed.

https://bugzilla.redhat.com/show_bug.cgi?id=1115986
g_type_init() is deprecated in GLib 2.36
Fix "REVERSE_INULL" caught by coverity
Prefer using g_malloc0()/g_free()

As we already depend on GLib, let's use g_{malloc,new}0() instead of the
standard malloc() or the spice_{malloc,new}*() and g_free() instead of
the standard free() when possible.
Memory allocated by other libraries using malloc() should still be freed
by free().
As a side effect of the changes, we are muting a few warnings caught by
coverity.
Don't use _GET_PRIVATE all the time

Let's make use of the priv members in the structs, which are much much
faster. Also, to keep the type-safety, the SPICE_IS_* macros were added
in the public methods affected by this patch.

Patch based on an old patch from Alexander Larsson (alexl@redhat.com).
Update spice-common