target/i386: sev: Do not pin the ram device memory region

The RAM device presents a memory region that should be handled
as an IO region and should not be pinned.

In the case of the vfio-pci, RAM device represents a MMIO BAR
and the memory region is not backed by pages hence
KVM_MEMORY_ENCRYPT_REG_REGION fails to lock the memory range.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1667249


Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Message-Id: <20190204222322.26766-3-brijesh.singh@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>