Tested and confirmed. Modifying the /etc/default/grub like below makes a grub menu appear during virsh console boot now.
# Kernel console on both serial and kvm/local console
GRUB_CMDLINE_LINUX="console=tty0 console=ttyS0,115200n8 rootdelay=60"
# Show grub menu on both serial and kvm/local console
GRUB_TERMINAL="console serial"
GRUB_SERIAL_COMMAND="serial --unit=0 --speed=115200 --word=8 --parity=no --stop=1"
Excellent. This might also help Whonix Host (when that progresses). When booting Whonix Host from iso (live mode) or installed in live mode, a script might instruct the VMs to boot in live too.