docs: add domain vmport feature

docs: add domain vmport feature

A new feature that can be turned on or off.

The QEMU machine vmport option allows to set the VMWare IO port
emulation. This emulation is useful for absolute pointer input when the
guest has vmware input drivers, and is enabled by default for kvm.

However it is unnecessary for Spice-enabled VM, since the agent already
handles absolute pointer and multi-monitors. Furthermore, it prevents
Spice from switching to relative input since the regular ps/2 pointer
driver is replaced by the vmware driver. It is thus advised to disable
vmport when using a Spice VM. This will permit the Spice client to
switch from absolute to relative pointer, as it may be required for
certain games or applications.