Port allocations for SPICE and VNC behave almost the same (with
default ports), but there is some mess in the code. This patch clears
these inconsistencies and makes sure the same behavior will be used
when ports for remote displays are changed.

Changes:
 - hard-coded number 5900 removed (handled elsewhere like with VNC)
 - reservedVNCPorts renamed to reservedRemotePorts (it's not just for
   VNC anymore)
 - QEMU_VNC_PORT_{MIN,MAX} renamed to QEMU_REMOTE_PORT_{MIN,MAX}
 - port allocation unified for VNC and SPICE