xen/pygrub: grub2/grub.cfg from RHEL 7 has new commands in menuentry

xen/pygrub: grub2/grub.cfg from RHEL 7 has new commands in menuentry

menuentry in grub2/grub.cfg uses linux16 and initrd16 commands
instead of linux and initrd. Due to this RHEL 7 (beta) guest failed to
boot after the installation.

In addition to this, RHEL 7 menu entries have two different single-quote
delimited strings on the same line, and the greedy grouping for menuentry
parsing gets both strings, and the options inbetween.

Signed-off-by: Joby Poriyath <joby.poriyath@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Cc: george.dunlap@citrix.com