Allow to use also special prefixed names for test

Allow to use also special prefixed names for test

Currently we could not test special prefixes in our test suite.
As teardown will not find such device and basicaly busyloops here,
as at cannot remove such names.

This patch adds possibity to use:

vgcreate  V_$vg1 $dev

Note: you still need to use $PREFIX somewhere in the name.
(And of course, it's really bad idea to use $PREFIX (=LVMTEST)
 for normally used LVs)

The only purpose of this patch is to allow testing cluster with
special vg names that begins with V_ , P_....