grub: add console=tty0 to kernel cmdline

When there is only console=hvc0 (i.e. no output to emulated VGA) and
GRUB_TIMEOUT is set to 0, VM startup hangs. This may be very well some
race condition broken by either of console=tty0 or GRUB_TIMEOUT > 0, but
even in such a case, apply this as a workaround for now.

misc/grub.qubes

@@ -2,7 +2,7 @@
 # same fs, including UUID
 GRUB_DISABLE_LINUX_UUID=true
 GRUB_DISABLE_OS_PROBER=true
-GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX root=/dev/mapper/dmroot console=hvc0"
+GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX root=/dev/mapper/dmroot console=hvc0 console=tty0"
 # make SWIOTLB smaller - it isn't really needed unless PCI passthrough is used,
 # and even then, 16MB is enough
 GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX swiotlb=8192"