Dynamic modes support for xenfb (0 of 2)

Patches 1of2 and 2of2 adds dynamic frame buffer resolution support to the PV xenfb frame buffer driver and the PV xenfb VNC server.

Don't know if you still need an API Changelog entry, included it here just in case.

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API Changelog entry:

PV framebuffer dynamic resolution facility:

Guest may send XENFB_TYPE_RESIZE if feature-resize = 1 in xenstore of the backend VNC server. VNC server code sets feature-resize if it can handle the resize request.

VM kernel module parameter array, xenfb.video, is used to set the amount of frame buffer memory in MB allocated and the initial width and height.

IE: extra="xenfb.video=3,1024,768 "

New xenstore virtual machine specific VNC attributes:
videoram: Sets the maximum allowable frame buffer allocation
size in MB. Protects against a hostile frontend trying to run dom0 out of memory. Not set by defualt

IE: vfb=['type=vnc,vncunused=1,videoram=3']

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