xen/pt: Remove XenPTReg->data field.

We do not want to have two entries to cache the guest configuration
registers: XenPTReg->data and dev.config. Instead we want to use
only the dev.config.

To do without much complications we rip out the ->data field
and replace it with an pointer to the dev.config. This way we
have the type-checking (uint8_t, uint16_t, etc) and as well
and pre-computed location.

Alternatively we could compute the offset in dev.config by
using the XenPTRRegInfo and XenPTRegGroup every time but
this way we have the pre-computed values.

This change also exposes some mis-use:
 - In 'xen_pt_status_reg_init' we used u32 for the Capabilities Pointer
   register, but said register is an an u16.
 - In 'xen_pt_msgdata_reg_write' we used u32 but should have only use u16.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>