x86/HVM: hvmemul_cmpxchg() should also use known_gla()

x86/HVM: hvmemul_cmpxchg() should also use known_gla()

To be consistent with the write and rmw cases the mapping approach
should not be used when the guest linear address translation is known.
This in particular excludes the discard-write case from bypassing the
emulation path. This also means that now EFLAGS should actually get
properly updated, despite the discarded write portion of the memory
access.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>