_require_command: quote commands correctly

_require_command: quote commands correctly

fixes 9435b92 common: _require_command needs to handle parameters

Also quoted $_command because _require_command may be called with an
empty $1 parameter, e.g.:
_require_command "$MY_UTIL_PROG" my_util  # but $MY_UTIL_PROG is empty

[ -x ] returns true.
[ -x "" ] returns false, as required here.

Signed-off-by: Omer Zilberberg <omzg@plexistor.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>