qemu-option: has_help_option() and is_valid_option_list()

qemu-option: has_help_option() and is_valid_option_list()

has_help_option() checks if any help option ('help' or '?') occurs
anywhere in an option string, so that things like 'cluster_size=4k,help'
are recognised.

is_valid_option_list() ensures that the option list doesn't have options
with leading commas or trailing unescaped commas.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>