Skip /dev/hpet during 'zpool import'

Skip /dev/hpet during 'zpool import'

If libblkid does not contain ZFS support, then 'zpool import' will scan
all block devices in /dev/ to determine which ones are components of a
ZFS filesystem.  It does this by opening all the devices and stat'ing
them to determine which ones are block devices.  If the device turns
out not to be a block device it is skipped.

Usually, this whole process is pretty harmless (although slow).  But
there are certain devices in /dev/ which must be handled in a very
specific way or your system may crash.  For example, if /dev/watchdog
is simply opened the watchdog timer will be started and your system
will panic when the timer expires.

It turns out the /dev/hpet causes similiar problems although only when
accessed under a virtual machine.  For some reason accessing /dev/hpet
causes qemu to crash.  To address this issue this commit adds /dev/hpet
to the device blacklist, it will be skipped solely based on its name.