KVM: VMX: Rename the "shared_msr_entry" struct to "vmx_uret_msr"

Rename struct "shared_msr_entry" to "vmx_uret_msr" to align with x86's
rename of "shared_msrs" to "user_return_msrs", and to call out that the
struct is specific to VMX, i.e. not part of the generic "shared_msrs"
framework.  Abbreviate "user_return" as "uret" to keep line lengths
marginally sane and code more or less readable.

No functional change intended.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200923180409.32255-5-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>