PCI: hv: Introduce hv_msi_entry

Add a new structure (hv_msi_entry), which is also defined in the TLFS,
to describe the msi entry for HVCALL_RETARGET_INTERRUPT. The structure
is needed because its layout may be different from architecture to
architecture.

Also add a new generic interface hv_set_msi_entry_from_desc() to allow
different archs to set the msi entry from msi_desc.

No functional change, only preparation for the future support of virtual
PCI on non-x86 architectures.

Signed-off-by: Boqun Feng (Microsoft) <boqun.feng@gmail.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Dexuan Cui <decui@microsoft.com>