qemu: Probe QEMU executable path

Add QEMU executable path probing to Capstan to support CentOS 6
out-of-the-box. Also add a CAPSTAN_QEMU_PATH environment variable which
can be used to override the path.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
nbd: Flush only if it's supported

Use the flags from NBD handshake to determine if we need to flush or
not. Needed for older versions of QEMU.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Merge pull request #134 from penberg/centos6

CentOS 6 support