target/i386/mshv: Integrate x86 instruction decoder/emulator

target/i386/mshv: Integrate x86 instruction decoder/emulator

Connect the x86 instruction decoder and emulator to the MSHV backend
to handle intercepted instructions. This enables software emulation
of MMIO operations in MSHV guests. MSHV has a translate_gva hypercall
that is used to accessing the physical guest memory.

A guest might read from unmapped memory regions (e.g. OVMF will probe
0xfed40000 for a vTPM). In those cases 0xFF bytes is returned instead of
aborting the execution.

Signed-off-by: Magnus Kulke <magnuskulke@linux.microsoft.com>
Link: https://lore.kernel.org/r/20250916164847.77883-21-magnuskulke@linux.microsoft.com
[mshv.h/mshv_int.h split. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>