Move common string instruction bookkeeping into the x86 instruction
emulator so MOVS, STOS, and MSHV port string I/O use one implementation
for REP counts, direction-flag handling, and index advancement.

This keeps existing MOVS/STOS behavior unchanged while removing the need
for MSHV to open-code the same string-operation details.

Signed-off-by: Wei Liu <liuwe@microsoft.com>
Assisted-by: Copilot:GPT-5.5

Decode MSHV port access size, direction, string, and REP state through
small helpers instead of open-coded bitfield reads.

Keep scalar I/O behavior unchanged and continue rejecting string I/O in
this step. Put the x86_64 port I/O helpers in their own module so the
string emulation path can grow without bloating the top-level MSHV code.

Signed-off-by: Wei Liu <liuwe@microsoft.com>
Assisted-by: Copilot:GPT-5.5

Handle MSHV INS/OUTS port intercepts by translating the guest string
operand through MshvEmulatorContext and copying data between guest
memory and existing PIO callbacks.

Support REP counts, zero-count REP, and direction-flag based RSI/RDI
updates. Commit RIP plus RCX/RSI/RDI after the transfer completes.

This removes the fw_cfg/debug-port skip. OVMF can now use the real
string I/O path instead of relying on ignored ports.

Signed-off-by: Wei Liu <liuwe@microsoft.com>
Assisted-by: Copilot:GPT-5.5

Re-enable the fw_cfg integration tests for MSHV now that port string I/O
is handled by the hypervisor backend.

Signed-off-by: Wei Liu <liuwe@microsoft.com>
Assisted-by: Copilot:GPT-5.5