qemu: Extract -mem-path building into its own function

qemu: Extract -mem-path building into its own function

That function is called qemuBuildMemPathStr() and will be used in
other places in the future.  The change in the test suite is proper due
to the fact that -mem-prealloc makes only sense with -mem-path (from
qemu documentation -- html/qemu-doc.html).

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>