ui: vm opts: clarify QGA option

ui: vm opts: clarify QGA option

To make it more clear that PVE does not somehow magically injects a
QHA into the VM, but that this can be set if one has installed the
QGA in the VM themself.

So slightly adapt the label by prepending "Use" and adding a hint for
further details.

Also ensure we call it the real name everywhere, i.e., "QEMU Guest
Agent", this makes it a bit more consistent and easier to find
information for users.

Suggested-by: Aaron Lauterer <t.lamprecht@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>