util: rework error reporting in virGet(User|Group)IDByName
util: rework error reporting in virGet(User|Group)IDByName

https://bugzilla.redhat.com/show_bug.cgi?id=883832

This patch gets rid of the undeterministic error reporting code done on
return values of get(pw|gr)nam_r. With this patch, if the group record
is not returned by the corresponding function this error is not
considered fatal even if errno != 0. The error is logged in such case.
(cherry picked from commit 96460a19870d4442b9dae3bb928ea0cd8d455369)

mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2,mithleshvrts/libvirt-0.10.2