slirp: Use monotonic clock if available (v2)

slirp: Use monotonic clock if available (v2)

Calling gettimeofday() to compute a time interval can cause problems if
the system clock jumps forwards or backwards; replace updtime() with
qemu_get_clock(rt_clock), which calls clock_gettime(CLOCK_MONOTONIC) if
it is available.

Also remove some useless macros.

Signed-off-by: Ed Swierk <eswierk@aristanetworks.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>