core: Extract common Custom Properties code

Moved basic validation from SimpleCustomPropertiesUtil to
CustomPropertiesUtils, as well as some validation from
VmPropertiesUtils that should be common to all custom properties.

Removed a lot of additional logic from VmPropertiesUtils that could be
accomplished using logic already existing in CustomPropertiesUtils (in
some cases after small modifications).

Change-Id: I24efa004b1b7fe4359046bca1e6c87188cececf9
Signed-off-by: Lior Vernia <lvernia@redhat.com>