Search for tap-ctl on PATH,XCP_PATH; pass through PATH, TAPDISK, TAPDISK2 environment variables

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Search XCP_PATH before PATH for tap-ctl

This allows an override to be provided.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #2 from djs55/blktap2.5

Allow for parallel installs of blktap2.5