blktap: re-enable blktap1 if blktap2 is disabled

blktap: re-enable blktap1 if blktap2 is disabled

This patch re-enables a useful blktap1 for users who disable blktap2.
Itremoves tapdisk and blktapctrl from blktap2, both of which cause
problems with blktap operation.  In addition, this patch modifies xend
to check for blktap2 installation.  If the blktap2 driver isn't
running (hopefully because the dom0 kernel option wasn't selected) we
fall back to blktap.

Signed-off-by: Dutch Meyer <dmeyer@cs.ubc.ca>