CA-142530: Use kernel page tables when in user context

Use vcpu->arch.guest_table to access the kernel page tables since the
cr3 register may point to user page tables.  This allows us to get the
kernel command-line and console log even when VCPU 0 is in user context
when the crash occurs.

Also, only dump the stack when in kernel mode.  Log the arch flags,
guest_table_user and guest_table for extra information.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>