KVM: arm64: Take any host SError before entering the guest

On VHE systems KVM masks SError before switching the VBAR value. Any
host RAS error that the CPU knew about before world-switch may become
pending as an SError during world-switch, and only be taken once we enter
the guest.

Until KVM can take RAS SErrors during world switch, add an ESB to
force any RAS errors to be synchronised and taken on the host before
we enter world switch.

RAS errors that become pending during world switch are still taken
once we enter the guest.

Signed-off-by: James Morse <james.morse@arm.com>