l4lx: raise block TX buffer size to 5 MiB

Buffer sizes lower than 5 MiB slow down the Arndale AHCI benchmark
with fast 6 Gbps SSDs and 1MiB blocks.
Use Genode's memcpy in L4Linux block driver
ahci & exynos5: compliant exceptions on creation

Use Root::Unavailable exceptions instead of driver specific ones
on session construction.