x86/emul: Use system-segment relative memory accesses

x86/emul: Use system-segment relative memory accesses

With hvm_virtual_to_linear_addr() capable of doing proper system-segment
relative memory accesses, avoid open-coding the address and limit calculations
locally.

When a table spans the 4GB boundary (32bit) or non-canonical boundary (64bit),
segmentation errors are now raised.  Previously, the use of x86_seg_none
resulted in segmentation being skipped, and the linear address being truncated
through the pagewalk, and possibly coming out valid on the far side.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <JBeulich@suse.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>