qemu_process: mark auto-generated spice ports as reserved

qemu_process: mark auto-generated spice ports as reserved

In case you will specify graphics like this:

<graphics type='spice' port='-1'/>

or

<graphics type='spice' port='-1' tlsPort='6000'/>

libvirt will automatically add autoport='no'.  This leads to an issue
that in qemuProcessStop() we don't release that port because we are
releasing both port if autoport=yes or only port marked as reserved.

If autoport=no but we request to generate port via '-1' we need to mark
that port as reserved in order to release it.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1299696

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>