docs: Remove redundant qemu-xen-security document

docs: Remove redundant qemu-xen-security document

All this information is now covered in SUPPORT.md.

Most of the emulated hardware is obvious a couple of the items are
worth pointing out specifically.

"xen_disk" is listed under "Blkback"

"...the PCI host bridge and the PIIX3 chipset...": This statement is
redundant -- the PCI host bridge is a part of the piix3 chipset, which
is listed as supported.

xenfb: The "graphics" side of "xenfb" is listed under "PV Framebuffer
(backend)", and the "input" side of "xenfb" (including both keyboard
and mouse) is listed under "PV Keyboard (backend)".

Backing storage image format is listed in the "Blkback" section.

Fix 'stdvga' spelling while we're here.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>