libqos: move common i2c code to libqos

The functions to read/write 8-bit or 16-bit registers are the same
in tmp105 and pca9552 tests, and in fact they are a special case of
"read block"/"write block" functionality; read block in turn is used
in ds1338-test.

Move everything inside libqos-test, removing the duplication.  Account
for the small differences by adding to tmp105-test.c the "read register
after writing" behavior that is specific to it.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>