Fix syntax error in GRUB_OPTS env var handling

Reference to ${GRUB_OPTS:""} in 'vm' element causes syntax error
if GRUB_OPTS is set. Fix by changing to ${GRUB_OPTS:-""}.

Change-Id: Iae3ee8b7c4064c92d0cfc1b5a8caa22e53d6d112
Merge "Fix syntax error in GRUB_OPTS env var handling"