x86, hvm: Fix issue with user-mode writes to read-only memory.

x86, hvm: Fix issue with user-mode writes to read-only memory.

This patch fixes an issue where a guest could get stuck if a write to
memory marked p2m_ram_ro happened from user mode. It would get
misinterpreted as a user-mode page-table write, and the "dummy write"
emulation necessary to skip over the instruction never got done. In
looking into this, I also discovered that the user-mode page-table
check is done in two places, the second (in emulate_map_dest) of which
can never be reached and is just a waste of cycles. Tim Deegan
requested that rather than completely removing that code, I'd leave it
in for debug-builds with an added warning-print.

Signed-off-by: Trolle Selander <trolle.selander@eu.citrix.com>