graphics: remember graphics not auto allocated ports

graphics: remember graphics not auto allocated ports

When looking for a port to allocate, the port allocator didn't take in
consideration ports that are statically set by the user.  Defining
these two graphics elements in the XML would cause an error, as the
port allocator would try to use the same port for the spice graphics
element:

    <graphics type='spice' autoport='yes'/>
    <graphics type='vnc' port='5900' autoport='no'/>

The new *[pP]ortReserved variables keep track of the ports that were
successfully tracked as used by the port allocator but that weren't
bound.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1081881
Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>