Emulate reading of the IA32_MISC_ENABLE MSR, by returning
the host MSR and masking off features that aren't supported.
Linux reads this MSR to detect if NX has been disabled via
BIOS.
On those machines, where sf_bufs do not represent any real object, make
sf_buf_alloc()/sf_buf_free() inlines, to save two calls to an absolutely
empty functions.

Reviewed by:	alc, kib, scottl
Sponsored by:	Nginx, Inc.
Sponsored by:	Netflix
Fix build.
Allow level-triggered interrupt sources. While this isn't
precisely emulated, it is good enough for the single consumer
i.e. irq4, the serial port on Linux.
Fix spelling.