KVM: selftests: Add coverage for 'b' (byte) sized fastops emulation

KVM: selftests: Add coverage for 'b' (byte) sized fastops emulation

Extend the fastops test to cover instructions that operate on 8-bit data.
Support for 8-bit instructions was omitted from the original commit purely
due to complications with BT not having a r/m8 variant.  To keep the
RFLAGS.CF behavior deterministic and not heavily biased to '0' or '1',
continue using BT, but cast and load the to-be-tested value into a
dedicated 32-bit constraint.

Supporting 8-bit operations will allow using guest_test_fastops() as-is to
provide full coverage for DIV and IDIV.  For divide operations, covering
all operand sizes _is_ interesting, because KVM needs provide exception
fixup for each size (failure to handle a #DE could panic the host).

Link: https://lore.kernel.org/all/aIF7ZhWZxlkcpm4y@google.com
Link: https://lore.kernel.org/r/20250909202835.333554-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>