OvmfPkg/QemuFwCfgLib: generalize InternalQemuFwCfgDmaBytes() to SKIP op
The fw_cfg DMA interface provides a simple method to skip over bytes in an
fw_cfg blob before reading or writing more bytes.
InternalQemuFwCfgDmaBytes() can support it easily, we just have to expose
the Control parameter more flexibly than the current "Write" BOOLEAN.