hvf: correctly handle REX prefix in relation to legacy prefixes

In real x86 processors, the REX prefix must come after legacy prefixes.
REX before legacy is ignored.  Update the HVF emulation code to properly
handle this.  Fix some spelling errors in constants.  Fix some decoder
table initialization issues found by Coverity.

Signed-off-by: Cameron Esfahani <dirty@apple.com>
Message-Id: <eff30ded8307471936bec5d84c3b6efbc95e3211.1574625592.git.dirty@apple.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>