target/i386/emulate: Allow instruction decoding from stream

target/i386/emulate: Allow instruction decoding from stream

Introduce a new helper function to decode x86 instructions from a
raw instruction byte stream. MSHV delivers an instruction stream in a
buffer of the vm_exit message. It can be used to speed up MMIO
emulation, since instructions do not have to be fetched and translated.

Added "fetch_instruction()" op to x86_emul_ops() to improve
traceability.

Signed-off-by: Magnus Kulke <magnuskulke@linux.microsoft.com>
Link: https://lore.kernel.org/r/20250916164847.77883-3-magnuskulke@linux.microsoft.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>