x86/vmx: don't clobber exception_bitmap when entering/leaving emulated real mode

x86/vmx: don't clobber exception_bitmap when entering/leaving emulated real mode

Most updates to the exception bitmaps set or clear an individual bits.

However, entering or exiting emulated real mode unilaterally clobbers it,
leaving the exit code to recalculate what it should have been.  This is error
prone, and indeed currently fails to recalculate the TRAP_no_device intercept
appropriately.

Instead of overwriting exception_bitmap when entering emulated real mode, move
the override into vmx_update_exception_bitmap() and leave exception_bitmap
unmodified.

This means that recalculation is unnecessary, and that the use of
vmx_fpu_leave() and vmx_update_debug_state() while in emulated real mode
doesn't result in TRAP_no_device and TRAP_int3 being un-intercepted.

This is only a functional change on hardware lacking unrestricted guest
support.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>