x86/emul: Require callers to provide LMA in the emulation context

x86/emul: Require callers to provide LMA in the emulation context

Long mode (or not) influences emulation behaviour in a number of cases.
Instead of reusing the ->read_msr() hook to obtain EFER.LMA, require callers
to provide it directly.

This simplifies all long mode checks during emulation to a simple boolean
read, removing embedded msr reads.  It also allows for the removal of a local
variable in the sysenter emulation block, and removes a latent bug in the
syscall emulation block where rc contains a non X86EMUL_* constant for a
period of time.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>
Reviewed-by: Jan Beulich <JBeulich@suse.com>