target/i386: rename hvf_mmio_buf to emu_mmio_buf

target/i386: rename hvf_mmio_buf to emu_mmio_buf

We want to refactor HVF's instruction emulator to a common component. Renaming
hvf_mmio_buf removes the association between HVF and the instruction emulator.

The definition of the field is still guarded by CONFIG_HVF for now, since it is
the only user.

No functional change.

Signed-off-by: Wei Liu <liuwe@linux.microsoft.com>
Link: https://lore.kernel.org/r/1741377325-28175-5-git-send-email-liuwe@linux.microsoft.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>