meson: src/util/virfile: rewrite virFileActivateDirOverrideForProg

meson: src/util/virfile: rewrite virFileActivateDirOverrideForProg

With meson we no longer have .libs directory with the actual binary so
we have to take a different approach to detect if running from build
directory.

This is not as robust as for autotools because if you select --prefix
in the build directory it will incorrectly enable the override as well
but nobody should do that.

We have to modify some of the tests to not add current build path into
PATH variable and use the full path for virsh instead. Otherwise it
would be impossible to figure out that we are running virsh from build
directory.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Neal Gompa <ngompa13@gmail.com>