xen/arm: vgic: Correctly emulate write when byte is used

xen/arm: vgic: Correctly emulate write when byte is used

When a guest is writing a byte, the value will be located in bits[7:0]
of the register.

Although the current implementation is expecting the byte at the Nth
byte of the register where N = address & 4;

When the address is not 4-byte aligned, the corresponding byte in the
internal state will always be set to zero rather.

Note that byte access are only used for GICD_IPRIORITYR and
GICD_ITARGETSR. So the worst things that could happen is not setting the
priority correctly and ignore the target vCPU written.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>