webadmin: removed ugly if-else chain

most of given 'rules' can be replaced by polymorphism.
Remaining ones(VmPool, DbUser and DbGroup) are probably only badly
created and should be fixed to remove this chain altogether.

Returning Guid.Empty is not apropriate and should be removed as well.

Change-Id: Ic716dc67b90c1d575de14c40b504c984bf0467f2
Signed-off-by: Martin Mucha <mmucha@redhat.com>