use --libexecdir instead of ignoring it first and reinventing it later

use --libexecdir instead of ignoring it first and reinventing it later

Commit 7b93fadf3a38d1ed65ea5536a52efc2772c6e3b8 "Add basic version
of bridge helper" put the bridge helper executable into a fixed
${prefix}/libexec/ location, instead of using ${libexecdir} for
this.  At the same time, --libexecdir is being happily ignored
by ./configure.  Even more, the same patch sets unused $libexecdir
variable in the generated config-host.mak, and uses fixed string
(\${prefix}/libexecdir) for the bridge helper binary.

Fix this braindamage by introducing $libexecdir variable, using
it for the bridge helper binary, and recognizing --libexecdir.

This patch is applicable to stable-1.1.

Reviewed-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Corey Bryant <coreyb@linux.vnet.ibm.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Cc: Corey Bryant <coreyb@linux.vnet.ibm.com>
Cc: Richa Marwaha <rmarwah@linux.vnet.ibm.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>