Passing tap devices to KVM as file descriptors requires that the respective
file decriptors remain open during utils.RunCmd execution. To this direction,
we add a “noclose_fds” keyword argument to utils.RunCmd, accepting a list of
file descriptors to keep open. The actual fd handling is implemented in
_RunCmdPipe and _RunCmdFile using subprocess.Popen's “preexec_fn”[1],
since subprocess.Popen provides no other way to selectively handle fds.

A small modification is also made to test/ganeti.utils_unittest.py to comply
with _RunCmdPipe's new API and a new unit test is added to test the selective
fd retention functionality.

[1] “If preexec_fn is set to a callable object, this object will be called in
     the child process just before the child is executed. (Unix only)”
    Subprocess documentation

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>