floppy0.present defaults to true. Therefore, it needs to be
explicitly set to false when the XML config doesn't specify the
corresponding floppy device.
Also update tests accordingly.
* src/conf/domain_conf.c: add defaults for the video device
* src/esx/esx_vmx.[ch]: add VNC support to the VMX handling
* tests/vmx2xmltest.c, tests/xml2vmxtest.c: add tests for the VNC support