kvm: add call to qemu_add_opts() for -overcommit option

kvm: add call to qemu_add_opts() for -overcommit option

qemu command fails to process -overcommit option. Add the missing
call to qemu_add_opts() in vl.c.

Signed-off-by: Prasad Singamsetty <prasad.singamsetty@oracle.com>
Message-Id: <20180815175704.105902-1-prasad.singamsetty@oracle.com>
Reviewed-by: Mark Kanda <mark.kanda@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 1fdd4748711a62d863744f42b958472509a6f202)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>