tests: Fix MinGW build for domaincapstest

Commit 5b9819eedc71 started using the virFileWrapper APIs in
the test program, and correctly called them only in the section
of code guarded by WITH_QEMU; however, a single call to the
virFileWrapperClearPrefixes() function ended up in the
hypervisor-agnostic section, causing a build failure on MinGW.

Move the call to the QEMU-only section; while at it, also drop
the virFileWrapperRemovePrefix() calls, which are entirely
redundant since we'd drop all prefixes immediately afterwards
anyway.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Acked-by: Michal Privoznik <mprivozn@redhat.com>