qemumachines: Enable xserver-xorg as default xserver
For qemux86 and qemux86-64 include qemu.inc after defining XSERVER
XSERVER variable is also weakly defined in task-core-x11.bb
which means we can not use ??= otherwise when building any qemu image
that uses task-core-x11.bb will get the wrong definition
So we define the XSERVER common set for qemu in qemu.inc
and as we know x86 and x86-64 qemu overrides the default
we include qemu.inc after that definition which means that
qemux86 and qemux86-64 get their own definitions and other
qemus get the definitions from qemu.inc. other non-qemu machine
will get their defintion from task which points to kdrive
as of now.
