Fix install(1) usage to be compatible with OpenBSD's install(1).

When creating a directory via the -d flag the -p flag cannot be
used at the same time. Also in the context of installing QEMU it
doesn't make sense to use the -p flag anyway so use the [default]
-c flag instead.
Signed-off-by: NBrad Smith <brad@comstyle.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>