hw/i386: add a separate region that tracks the SMRAME bit

hw/i386: add a separate region that tracks the SMRAME bit

This region is exported at /machine/smram.  It is "empty" if
SMRAME=0 and points to SMRAM if SMRAME=1.  The CPU will
enable/disable it as it enters or exits SMRAM.

While touching nearby code, the existing memory region setup was
slightly inconsistent.  The smram_region is *disabled* in order to open
SMRAM (because the smram_region shows the low VRAM instead of the RAM
at 0xa0000).  Because SMRAM is closed at startup, the smram_region must
be enabled when creating the i440fx or q35 devices.

Acked-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>