Older dash mistakenly truncates regular files when using <> redirection;
this kills our use of double dd to reduce storage overhead when
saving qemu images.  But qemu insists on running a command through
/bin/sh, so we work around it by having qemu run $sh -c 'real command'
when we have a replacement $sh in mind.

* configure.ac (VIR_WRAPPER_SHELL): Define to a replacement shell,
if /bin/sh is broken on <> redirection.
* src/qemu/qemu_monitor.h (VIR_WRAPPER_SHELL_PREFIX)
(VIR_WRAPPER_SHELL_SUFFIX): New macros.
* src/qemu/qemu_monitor_text.c (qemuMonitorTextMigrateToFile): Use
them.
* src/qemu/qemu_monitor_json.c (qemuMonitorJSONMigrateToFile):
Likewise.