[CentOS-virt] What is the purpose setting console=hvc0 in the dom0 grub config?

Howdy,

I recently went through a frustrating experience trying to get Xen 4
running on a CentOS 7 system.  After a fresh install, fully updating the
system, rebooting, then trying to install Xen4CentOS it would fail to boot
into the 4.9 kernel, sitting there with a blinking cursor indefinitely.

I thought it was a failure with grub, but it turns out this was set in the
grub config:

GRUB_CMDLINE_LINUX_XEN_REPLACE_DEFAULT="console=hvc0 ..."

So the kernel was loading successfully, but it was failing boot properly
(hanging because it couldn't find the / file system, but that's a different
story). Boot messages were not being displayed on my console because of the
console=hvc0 setting, hampering the troubleshooting process.

What is the purpose of setting console=hvc0?  I removed it so I could
trouble shoot this problem, but I didn't re-add it and the system booted
fine, Xen is working.  Is there something that depends on this?

I always disable "rhgb quiet" on a fresh install because I don't like boot
messages being hidden from me, and now this other thing does it.  I like
details, I need the details, don't hide them from me.

Thanks,
Jerry
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